How do I use my old hard drive with my new build?

Unfortunately, there's no legal way to do that if your OS license is an OEM version. If you have a retail version, it should not be a problem. If your old/current computer is a pre-built from Dell or wherever, it is almost certain that you have an OEM copy.

Microsoft licensing is tied to the motherboard, not to the hard drive, with the assumption that if you change the motherboard, you are essentially creating a new system.

When building a new computer and reusing an old boot drive, you are best off reinstalling Windows, in order to avoid any issues with old and new drivers conflicting. If you have a retail copy, there should be no issue reinstalling XP on the new setup.

I have the disk but I'm not sure if it's a normal retail version or just an oem version. Is it possible to deactivate windows on my old computer and then reactivate it on the new one?

Is the disc a Windows install disk with the license key on it? Or does the label say something like "System Restore DVD"?

Its an install disk, so I guess it also has the license key on it

Your best bet in that case is:

1) take a backup of your current system or copy important files to a different physical drive (like an external USB drive or something)
2) move the hard drive to the new system
3) boot into the BIOS & set the CD/DVD drive as the first device
4) put the Windows install disc in the drive
5) reinstall Windows - if you format the disk, you will need to restore any files from the backup or alternate disk. If you choose to not format the disk, Windows should move all the files into C:\Windows.old or something similar.
6) Use the license key provided with your retail copy to validate Windows.

ok thank you, I'll give that a try tonight and post the results back up here.

Well I did what you said and windows installed fine and I kept all my files but the activation didn't work. O well I guess I'm just gonna buy a new copy of xp when the 30 days runs out.

I ran into another problem though, the boot time on this system is REALLY long, which I don't mind too much.

But the last problem I ran into is kind of important and I'd like some advice. I can't connect to the Internet, at all. I tried setting it up manually by entering the user name and password and that didn't work. I also tried logging into my modem by plugging in the Ethernet cable and typing my ip address into Internet browser address bar but that didn't work either.

I think there might be a problem with my motherboard because it's an old board and it seems like the pc doesn't even recognize that there is an ethernet cable plugged in. Drivers maybe?

For the ethernet problem, I would check drivers, yeah.

For the activation, you can try calling Microsoft and tell them what happened, and it's likely they'll give you a new key if everything checks out.