Can you boot into safe mode? Press F8 repeatedly when the PC is powering up to get to the menu to reach Safe Mode. This can happen in some rare cases if you installed OneCare with another antivirus program still installed and active.
A description of the Safe Mode Boot options in Windows XP
If you are unable to do this, please contact support for help.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve