Before reinstalling windows, you should do a repair....
The first thing to do is backup your files...
Back-up your "my Documents" folder, "Favorites", any Outlook Express e-mails, and any saved files you want to salvage..
Then to do a non destructive reinstall of windows...
boot to the XP cd
on the first screen hit "enter" as if for a fresh install (DONT choose (R) on this screen as it is the "recovery console" and is not what you want)
Skip the first repair option...
Choose the 'press enter to set up Windows XP now' option.
Press F8 to skip through the EULA.
At this point it will check for a previous install of windows and offer another repair option
Now press R to begin the repair installation.
Your system will go through the entire XP install process, but will not attempt to replace any of your existing data. It will simply reinstall the system files and redetect all hardware.
It WILL however lose your favorites and any Outlook Express e-mails
Once the process has completed, your computer will reboot and you can reactivate Windows, update drivers and make sure all Antivirus and Antimalware software is installed and updated again
You should re-download and install AVG