Left moving Taskbar and Transparent start menu

Hi,

When I leave my laptop alone for a while, any windows on the taskbar move to
the left and also the start menu goes transparent. the task basr is still
operational to the effect that if i click on it the program or window does
open up.
The only solution is for me to then log of and log back in which is really
irritating.

i use Vista home edition

any solution offered would be much appreciated.
Thanks