Problems Logging On/Off

Hey,
There are 4 users on my pc, (including me) and for some time now 3 of them have had problems logging off.
Basicly when you click start and log off, nothing happenes. We have had to press Ctrl Alt and Delete to log ourselfs off...
Just recently the computer has had problems starting up too. Sometimes it gets to the loging on screen, but just freezes

How do i fix this?

My computer is An Advent T9001 running on windows XP.
Intel Pentium 4 3.0 GHz Ht
512 MB DDR RAM
120 GB Hard drive...

Thanks, Josh.

 

Doesnt matter, fixed it now.
There was a conflict in Style XP, so i removed style xp and everythings fine.

Admin, feel free to lock this topic.