It sounds to me like there may be a problem with .Net Framework 2.0 which is required by OneCare to operate properly.
On XP, in Control Panel, go to add/remove programs and find Microsoft .Net Framework 2.0 in the list of programs. Select it and when the uninstaller loads, select repair. Reboot when finished.
If this does not resolve the problem, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve