no worries, it is actually pretty easy. all you have to do is physically put your drive in your new tower hook up the ribbon cable (ata 40 pin cable) , the power cable , and make sure the jumpers are properly set!!
then make sure the bios is set to auto detect.
now, heres some help doing that
1.ribbon cable>you shoud have two ribbon cables that are the same going from your motherboard, one goes to your hard drive, the other goes to your atapi devices (cd rom, dvd rom, optical drives). on each cable , there is 2 places to attach an ata (hard drive or atapi) device. i would put your old drive on the same cable as your new hard drive, on the middle connector of the ribbon cable.
2. power cable, not a major issue, simply plug it in.
3.jumpers, now this MAY be easy, or difficult. i dont know what drive you have so you will have to find the diagram for yourself, it is a diagram for jumper settings, the diagram should be on the harddrive. you will be looking for the "slave" setting. it should tell you how to put jumpers on for ..master..slave..cable select..etc. find the jumper setting for "slave", and move the jumpers to match it.http://www.mysuperpc.com/hdu/jumper_pins.shtml
there is a picture of what i mean.
4.now you are almost there..just get into the bios, when you start your computer, you have to hit a key to get into the setup, on a dell i think<but i cant remember but it will tell you so watch for it> i think it is f-10 or f-2.. so get into setup, and the first tab (depending on the bios) should get you into the screen you need. look to see that the ide 0 or ide 1 has picked up your old drive on the slave setting. you wil see the make and model indicated on ex:ide 0 slave .....if you see it reboot your computer into windows and go into "my computer" your hard drive should be there. give it a minute once you boot into windows for your operating system to find and install the drive. and viola..
this is a quick guide, hope it helps, because there is a few variables you may not get it the first try, just post back for more detailed instruction if you run into anyh difficulties