First, you will need to be more specific on the model number. As you can see from this page, A1278 actually refers to a number of different models/configurations (Apple's model number/naming system is rather a pain in the rear, IMHO):http://www.everymac.com/ultimate-mac-lookup/?search_keywords=a1278
Best way to look it up is by the serial number here:https://support.apple.com/en-us/HT201300
The first page does give the general "gist" of what is needed...i.e. does it have an optical drive, which appears to be the case with all versions associated with A1278.
The other reason for being more specific is that some of those models should have the Internet Recovery option available on them. If I recall correctly, any Mac that shipped with Lion (aka Mac OS X 10.7.x) and also some Macs that shipped with Snow Leopard (aka Mac OS X 10.6.x) could have Internet Recovery added when updated to Lion and a software/firmware update was run. The point is that a number of thos versions on the first link's list shipped with Lion, so if you have one of those, you likely have access to Internet Recovery.
So, the first thing to try is to see if you have Internet Recovery available to you. So, boot up the Mac while holding down the "Command" and "R" keys. I don't know if the drive needs to be installed at this point or not. If you have Internet Recovery, it will boot up and ask you to log into a WiFi network (assuming you are not plugged into a network by way of the ethernet port). With WiFi or ethernet connected, you should then be presented with the full array of utilities and options available to re-install the Mac OS (it will be the version originally shipped with the Mac). From there, you should be good to go to install the Mac OS. FWIW, this page might help:https://support.apple.com/en-us/HT201314
If you do not have Internet Recovery (there some Macs on the list in the first link that shipped with 10.5.x that will definitely not have it and some that shipped with 10.6.x that may or may not have it), then your next easiest option would be to buy or borrow a Mac OS optical install/recovery disc (or maybe a install/recovery USB drive). The difference between an install disc and a recovery disc are the following:
- An install disc (or maybe a USB drive...generally they will be optical discs) was the retail version of the Mac OS that your could purchase for upgrade purposes. It can be used to install the Mac OS on ANY Mac that supports that version of the Mac OS assuming that version is newer than what originally shipped with the Mac.
- An install disc will have a colorful image on the face of the disc like these: http://core0.staticworld.net/images/article/2014/02/macosxinstalldiscs-100247528-large.png
- A recovery disc (or USB drive (just before Apple stopped supplying recovery media and switched to the Internet Recovery and a recovery partition around the time of Lion, they shipped some Macs with USB drives rather than optical discs...this was generally because those models did not come with an optical drive...this was true of my first MacBook Air from 2010) is specific to the model/version of the Mac that it shipped with. In other words, it will ONLY work with a specific Mac model/version.
- A recovery disc will have a boring gray label on it, like this: http://www.oldapplestuff.com/Images/InstallSoftware/IMG_7606.jpg
You can buy used install/recovery discs thru various sites (including eBay). Or if you know someone with a Mac, there is a good chance they might have a retail install disc that you could borrow. As I said, if the Internet Recovery option is not doable, then this is likely your next easiest option.
The other overall potential easy option is to take the MacBook Pro to an Apple Store and see if they will install the Mac OS on the new hard drive in the Mac. Frankly, this could be the easiest option...assuming you have an Apple Store some what near you and that they will do this (I have to believe that they will, but they might charge something for it).
That brings us to trying to create a bootable install drive on your Windows computer. There are a number of reasons I left this to last.
First of all, I have never specifically tried to do this, so I don't know if it is really possible or not. I have to believe that it is possible as you can find pages of people claiming to do it. So, either they are lying or it is doable. Plus, I have to believe that some enterprising person has found a way to do it. But, the big point is that I have never done it, so I don't have any specific thoughts to provide at this point. If find some time, I might try to do it, but I cannot promise anything and even if I do, I don't know how quickly I can get to it.
Second, there is the question of whether or not your DMGs are of an actual bootable disc/disk. Apple stopped providing bootable media for the most part around the release of Lion. This was due to two things: 1) Apple started doing their Mac OS major version upgrades through the Apple Store; and 2) they went to a combination of an recovery partition on the hard drive/SSD and Internet Recovery for reinstallation of the Mac OS rather than shipping Macs with recovery discs (or USB drives) and retail optical discs. As a result, Mac OS X 10.7.x and newer was rarely available on a bootable media (you could [and maybe still can] buy USB drives if you wanted). The point is that for versions 10.7.x and newer, any default "installer" that you had was NOT bootable...you had to make a bootable disc/USB drive using instructions you could find on a number of web sites. The whole point of this ramble is that Snow Leopard DMG is very likely to be from a bootable disc, it is less likely that the Mountain Lion DMG is. AND if the DMG is NOT from a bootable drive, then things get even harder. Assuming at least one of the DMGs is of a bootable Mac install drive, then in theory all you would need is a Windows program that write a disk image (aka the DMG) to an optical disc or USB drive using the Mac OS drive format. I have to believe there is something out there that can do that (from some web searches, it appears that a program called TransMac can supposedly do this). That is what I would try to do IF I find some time. At a minimum, you likely could use a VM program to install the Mac OS in a VM on your Windows computer and then use that installation of the Mac OS to "burn" the bootable DMG to a disc or USB drive. This is something that should be very doable (although technically it violates Apple's EULA...whether or not that EULA is enforceable remains to be seen), but again I have never done it as I actually have Macs running the Mac OS, so I have no need to put the Mac OS on a Windows machine.
If it is not a DMG of a bootable drive, then you also need something that would also covert the non-bootable installer into something bootable in addition to somehow creating a Mac formatted disc or USB drive on a Windows computer. In other words, a MUCH more complex option.
So, I am not much help (at this time at least) on using Windows to get a bootable Mac install disc. It is possible that someone else might be able to help.
One other quick note, if you want that license of Windows 7 to run on the Mac, you can ONLY do that once you have the Mac OS installed. At that point, you can use Boot Camp to install it on another partition of the drive or you could use a VM program such as Parallels, VMWare or VirtualBox to run Windows 7 in a VM "inside" the Mac OS (i.e. no reboots needed to switch to Windows).
The other quick note...there should be no reason to hook up the hard drive to the Windows computer using an external enclosure. As implied by much of the above, the process you really want to do is get a bootable Mac OS install/recovery disc or USB drive that you can use to boot up the MacBook Pro with the drive installed. That will then allow you to reformat the hard drive and install the Mac OS...all on the Mac. As I said, I kind of implied it above, but wanted to explicitly state it. I am not sure there is ANY way to install the Mac OS on an external drive hooked up to a Windows computer...except by having the Mac OS running in a VM on that Windows computer and attaching/accessing the external drive through the Mac OS in the VM running "inside" Windows.
Hope this helps some.