If your CPU usage is spiking at 100% then it is likely that it is the result of a flawed Windows Security Update KB835732 which you have downloaded. Microsoft is aware of the problem but has kept relatively quiet about it as issuing a patch for a patch is not good PR.
http://support.microsoft.com/kb/841382/
The above link will detail how to unpatch the patch for Windows 2000. The strange thing about this Windows Security FLAW is that it doesn't affect every computer on which it has been installed.
It would also be a good idea to download Lavasoft AdAware SE, Spybot: Search & Destroy, and HiJack This. All are very good at keeping your computer clean and under your control. Switching from Internet Explorer over to OPERA will also reduce many of the security issues associated with IE.
Hope this helps you get back to normal.
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IE_Sux
October 2005