Nvidia 7900 GT not functioning in games ?

iv had vista for about 6 to 8 months and have done a clean installation to
try and see what was cuasing my problem which is that ever time i play a
game ie #halo 2 as soon as the start up screen loads all i get is lines of
various colours across the screen and to get out of that i have to press
keys blindly to get back to the desktop which is then the same so i have to
log out - switch user (cuz if i dont i get the same garbbled screen) and
then log back in every thing is fine until/ if i try to play Halo 2.
i did the clean install so i could monitor from scracth wath updates for
windows where installled and second time round only use original drivers
from manufacturere which i did all microsft updates installed but using the
nvidia driver that came with windows vista everything is fine i can play
Halo 2 to my hearts content maybe the odd crash now and then and its an
nforce mother board with all the driver for same from nvidia but when i
install the latest nvidia 7 series driver (163.75) the same garbbled screen
appears. has anybody any ideas, and there is no error report generated for
this cuz the comp. oesnt crash so im totally lost. am going to go back to XP
if i cant sort this out before christmas have been trying for 6 month on to
microsoft and nvidia no replies or fixes and all nvidias drivers over that 6
months have im getting the same problem

Somebody hats had enough of vista !