Installing a hard drive is pretty simple: Follow TBone2’s advice about polarity (really, just match colors) and decide exactly what you want to do with your data, and then just crack the case.
A standard Phillip’s head should be more than sufficient for the whole process, from opening the case to screwing in the second drive. PCs, unlike older Macintoshes, are pretty much standardized when it comes to screw types. I’d be rather surprised to see a PC case I couldn’t open with my trusty screwdriver.
Anyway, you probably won’t want to immediately remove the first drive. First, it’ll be a lot easier to move data if both drives are active at the same time. Second, the new hard drive will not have an OS on it. That means your computer will have nothing to boot unless you happen to have installation disks handy.
Before putting the disk in the machine, look at the little notes printed on the side of the disk. They will show graphically how to set the jumpers to make the drive a master or a slave. A slave is what you want, at least for now: The drive has no OS on it and it will be controlled by the software on the master drive you already have. You might need tweezers, and be sure to do it over a table or something so you don’t lose the little jumper in carpeting. Don’t break the pins, as that would probably be rather embarassing. The rule is `gentle but firm.’
It not being practical to remove your current drive is not a problem. PC cases generally have plenty of room for another hard drive. Open the case and see: You’ll have at least one empty slot and two unplugged cables, one for power (small and white) and one to control the drive (wide with the single red wire). That’s all you need. Insert the drive to match your present drive, using the screws that came with the drive, and close the case. Be sure to push those cables in snugly, but you shouldn’t have to force anything. The power cable’s plug will be shaped so you can’t insert it backwards, for example, and you don’t want to mess with that.
It’ll be cramped work, maybe, but it shouldn’t be difficult.
Anyway, close the case again and plug everything back into the case. Turn it on and your OS will boot, most likely detecting the new drive right off. Let the OS format it (this might take a little time, but probably not long) and you’re good to go. You can move whatever files you want to save to the new drive.
I don’t know about moving the OS, though. You’ll still need the first drive, as that contains the OS and everything else that’s essential. That shouldn’t be a problem, actually. (I’m running Linux mostly the same way, in fact, although my second drive is 120 gigs. :D) If you want to move Windows XP, you’ll have to ask someone else. I don’t know how.