Could be any number of things. First, apply the most recent service pack (SP2 as of now) and check windowsupdate for any critical patches. Browse it for any other updates that seem relevant to your setup. If this doesn\'t help, then you\'ll need to download msconfig from the downloads section here, and see what all is being run at startup. Basically start unchecking things you don\'t need to be run every time you start up.
Unlike Win98, I don\'t think you can uncheck something critical to the system in the startup tab of msconfig in Windows 2000. All the critical processes are run as services. Of course if I turn out to be wrong, and you uncheck something that keeps your PC from booting, just boot to safe mode, run msconfig, and recheck the last thing(s) you unchecked, and reboot again.