Can you get a ping from any site?
Start->Run, type in "cmd" (without quotes).
When the command window appears, type in:
ping www.google.com
If the ping command doesn't time out, then it means that you've still got an internet connection.
Have you tried going to Windows Update while you still have an internet connection?
If you can't do this, then you can always manually install the Updates by downloading them from the Microsoft site and searching for Updates as of the last day that you updated. Download them onto a CD or USB key and run them on your computer.
Last thing to try:
Put your Windows XP CD in the CD-ROM drive.
Start->Run, "cmd"
type in:
sfc /scannow
sfc stands for System File Checker. It will check the integrity of your system files against the files that are on the CD and replace any that aren't Microsoft compliant (i.e. files that have been damaged/replaced by a virus).