Backup important files/contacts/programs/etc.
Assuming you have windows XP:
Locate XP disk.
Locate the Windows XP Product key (25 digit). Depending on your computer, it could be on the XP CD case, or if your computer came with XP it could be on the computer tower itself. Make sure you find this first, I didn't, and was freaking out with a useless computer infront of me frantically trying to find it. It was a laptop and it was hidden on the bottom
This probably explains everything better than I ever could.
Another thing, make sure you know all of your windows passwords. I don't think the link mentions it, but it asked me for a password on all three computers I tried. It is a security measure to make sure people don't delete everything. If you haven't set a PW, then you should be able to leave the prompt blank and press enter. For some reason this didn't work on PrincessJeanie'* computer, so after 2 hours of trying to hack it, I can't recall what I actually did to get around it. If you have this problem, I can try to figure out what I did.
Lastly, I in no way am certain that this will help your problem. If it is a network connection problem...this may do nothing if it is a problem with your ISP, or it may fix it if it is a network setting in XP that needs to be "reset." THIS IS A LAST RESORT!
Any questions...lemme know.