I just checked Event Viewer and for me it shows me a lot of warnings.
Each warning contains:
The Desktop Window Manager is experiencing heavy resource contention.
Scenario : Video memory resources are over-utilized and there is thrashing happening as a result. Reducing number of running programs and open windows may help resolve this condition.
My videocard is OC'ed quite a lot. Could that be the cause?
I got a (old I know) GTS450 running at 968/1936/2075