Reboot tapping F8 each second.
Select Safe Mode at the menu.
Try System Restore to a date/time before this started:
Start/All Programs/Accessories/System Tools/System Restore
If that fails, from the same menu, choose "Last known good
configuration"
Otherwise a Repair Installation may be necessary:
Repair Installation:
http://www.dougknox.com/xp/tips/xp_repair_install.htm
Windows Updates will need to be reinstalled.
Data should be safe, back-up important data just in case.
Install Service Pack first and by itself.
Then install ALL Critical Updates no more than 4 at a time,
Disconnect the network/modem cable before starting repair and do NOT
reconnect the cable until repair is completed and firewall is
installed/enabled.
http://support.microsoft.com/?kbid=283673