When computer starts to boot the above message appears on screen & system shuts down.3 people need an answerI do too

c000021a {fatal system error}Windows Logon Process system process terminated unexpectedly with a status c0000135 System has been shut downI use PC Tools, and it gave me message to clean some files, and then to restart the computer. I was in the middle of working, so I didn't do it right away. When I did close all programs and restarted the computer, windows logo showed and then this message. I tried F8 and restore to last settings, but it didn't work. I use Windows XP Service Pack 2. My CD drive works. I have only Reinstallation CD that came with the computer.