vista intermittently switching between active application and desktop

vista home prem 64bit all updates current
asus p6t deluxe v1
core i-t 920 no overclock
12 gigs 1600mhz ram( at 1333mhz) corsair dominator
evga gtx 260 core 216 oc driver 195.62
velociraptor 150gb os drive
wd 1TB data drive
onboard audio
avg9 free
comodo firewall


A few days ago i noticed that i would be in a game or watching a movie and vista would switch/crash back to desktop out of whatever i was doing. I then click on the window/application on the taskbar to reenter it and i can resume either the game or video no problem.
was wondering if maybe a third party application like abobe could be causeing this?

i updated the video driver still does it
i changed performance for Visual effects still does it
i checked all programs in the firewall exceptions and other settings still does it

what is causing this ? any ideas would be appreciated

yeah i have that too, in flight sim and GTA iv

i can't figure it out , and its driveing me nutz

no one has any ideas?
sigh

ok problem fixed
either when i updated the driver for my marvel miniport ( driver for networking ) to 11.22.4.3
or i turned off yahoo search protection auto update

one of these was the cause as since doing it the problem has not reoccurred yet