How much room off my harddrive would another partition of Windows take? Roughly?
Depends on the version of Windows you are using as well as what features you install for Windows. It will also depend on overall size of the drive as that can potentially affect the size of the "blocks" the drive uses (the short explanation without any "techno-mumbo-jumbo" speak is that files can potentially take up more room on a large drive than on a smaller drive due to how the drives & formatting actually work). It will also depend on how much space you need for XBMC.
I can say that my Vista install (on its own separate drive) for my desktop computer uses about 35 GB. This does include a few programs and files, but not that much. My Vista version is a 32 bit Home Premium. It is on an 80 GB drive. The listed minimum recommended drive space on the box is "a 40 GB drive with at least 15 GB of free space available" (I would NOT recommend 15 GB, however).
For my Windows 7 install (again on its own separate drive), which has less things loaded on it than the Vista drive, is about 30 GB. It is 64 bit Windows 7 Professional. It is on a 150 GB Velociraptor drive. The listed minimum recommended is 15 GB for 32 bit and 20 GB for 64 bit...again I would certainly not recommend the minimum.
For my one XP install that I use for gaming with minimal other things installed (again on its own separate drive), it takes up about 25 GB. It is 32 bit Windows XP Pro. It is on a 160 GB drive.
I would say you likely do not want to go below 40 GB, but you might be able to squeeze it down a bit further.
And if you could refer me to an informative harddrive partitioning tutorial or something along those lines, that would be fantastic.
Here is a guide for re-sizing/shrinking existing partitions:http://www.bleepingcomputer.com/tutorials/shrink-and-extend-ntfs-volumes-in-windows/#shrink
Here is a guide for actually then creating/formatting the partitions:http://www.bleepingcomputer.com/tutorials/partition-and-format-a-disk-in-windows/
The above are using Windows built-in utilities. Note that for Vista, I believe it will do non-destructive partitioning (i.e. in theory, it should not wipe your whole drive)...but I still STRONGLY
recommend backing up the drive even when doing non-destructive partitioning.
Here is a guide if you want use GParted:http://www.bleepingcomputer.com/tutorials/resizing-adding-partitions-with-gparted-live/
Overall, I personally prefer to install different OSs on different drives. And I usually do the installation of the new OS with all other drive disconnected. This eliminates me having to monkey with a Boot Manager as it is usually a pain in the rear to "fix" the Boot Manager if you ever remove an OS. By doing it the way that I do it, I just remove the drive. Of course, my method requires you to be using a desktop with space for multiple drives (or maybe a laptop that can accommodate two drives).