Startup problem!

Im currently using AMD Athlon Thunderbird 1.3ghz with Windows 2000. Lately its been having start up problems, everytime I turn the computer on it doesn’t startup at all it stops at “verifying pool data” or sometimes when it gets to “windows 2000” itjust freezes there and doesn’t log onto the desktop
I have to restart the computer like 50 times for it to load…. what could be the problem? and how can i fix this??? Please help!!