Task bar freezes

Hi all, I've got a problem with the task bar (Windows Vista x64). 90% of the
times the task bar freezes as soon as I click on the start menu, while if I
click on other items it continues to work fine. The rest of the system
continues to work just fine except the control panel which also freezes
(differently from the task bar it is freezed also before I click on the start
menu)
This forces me to reboot the computer as long as the problem doesn't happen
(I have to do this about 10 times a day (( ) I'm sure I have no viruses on
the computer as I've got norton 2007 suite, windows firewall, data execution
prevention and user account control on. I've tried to restore old backup
points but with no luck.
Is this a known problem? Can I do anything? (other than "format C:/" of
course )) )