Ok, im confused about installing a new Harddrive. So you take the old one out, replace with the new one, then just turn the computer on? Or am I missing some fundamentals here?

What kind of drive is it? IDE/PATA or SATA?

I'm pretty sure the one that fried was IDE, and Im looking into getting another one. I heard SATA was the way to go, so I guess SATA

Do you have SATA compatibility on your motherboard? Otherwise, if you want SATA you'll need to buy an adapter card also.

How can I tell if its compatible or not?

A bit of an update here:

I put tried putting the old(guess it wasn't fried) HDD in one of my computers, and it seems to run fine as a slave, but it won't be a master. So, I just ordered another IDE drive for cheap and now back to the original question. Can I just pop the new drive in with the XP cd in and go? Or am I missing something?

Well your slave/master issue is probably because you are running it on the same IDE channel as another master. If the drive works in another computer, more likely than not something is wrong with the old IDE cable(s) or the old motherboard. Try using the IDE cable from your current computer, setting the old HDD to master and setting it up in the old pc (use a different molex/power adaptor if there's another one in the case, just to see). If it still doesn't work, there's a good chance that the old motherboard is broken and you might need to replace it before any hard drive will work on it.

You can also test it by plugging in your old pc's optical drive to where the hard drive used to be plugged in to see if the IDE channel is out, that's probably easier since you know the optical drive works as is in the old computer.