In case you didn't get automatic notification, Microsoft released a high-priority update for XP on 4-3-07. Since they normally release updates on 2nd Tuesday of the month, this one must be important if they released on the 1st Tuesday in April.

You can find it here:
http://www.microsoft.com/technet/security/Bulletin/MS07-017.mspx

You have to reboot after it's installed.

There are some known issues with this update related to Realtek HD Audio Control Panel which you may want to check out, if you use Realtek software:
http://support.microsoft.com/kb/925902

Ron