This problem has several symptoms but basically what is happening is that when a user logs in, they get no desktop icons, no start menu, and no task bar.  If you do a CTRL+ALT+DEL, you will see that explorer.exe is not running, but you can do a File->New Task and start it.  Once started, everything is normal.

You may login and the computer just logs you out immediately without a chance to do the CTRL+ALT+DEL and task manager thing.

In either case, the solution is the same.  The problem is due to a missing userinit.exe file.  This file should be located in C:\Windows\System32 (if you are in XP).  You can restore it from your operating system CD by either starting explorer and browsing your i386 folder on the disk or, if your computer is logging you out before you can do that, you will have to boot from the CD and use the recovery console.

All the best!

Luke