Frozen Start menu & Taskbar WinXP

Hello, I'm getting a problem on user logon with Windows XP. When logging on, the desktop freezes completely, and will not allow further progress. This is true for 4 out of 6 users. The other 2, can open programmes and explorer windows, from the desktop icons or from My Computer, and navigate using Alt+Tab, but the taskbar and start menu, when passing the mouse over, just show the hourglass, and will not activate.
I have the latest updates from symantec and AdAware installed and run, but no virus has been found, and no adware that ADaware could not delete. This makes me think maybe that some OS file has been deleted?
Any clues? Thanks