CPU running at 100% would point to an issue with a program starting up or a virus. Can you boot from an XP CD and re-install, choose to leave the files intact and only remove the Windows Folder?
If you can't boot to Safe mode then you are stuck for ways around it
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If you can use your login then something on the other profile is knackered, can you get into the start up menu for that profile or use this
http://www.mlin.net/StartupCPL.shtml
to stop anything running from startup on that profile which could be causing the problem? If you are an admin try checking the admin tools, event viewer for issues.