Did you uninstall Windows Live OneCare? Do you have other security software installed? How long ago did your OneCare subscription expire?
"Will not boot up" means what, exactly? Nothing happens when you press the power switch? The Windows logo appears, but Windows will not start? Windows tries to start? Any errors?
Without more details, I can only direct you to support -
How to reach support (FAQ) -
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
or the XP forums as I don't think that the uninstall of OneCare would cause this. Select the System Repair and Recovery topic:
http://social.answers.microsoft.com/Forums/en-US/category/windowsxp
-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~