Ok i have found an workaround for this.. hope this helps..
my previous os was vista, lappy: acer aspire 5738g, intel t6400, nvidia geforce g105m cuda 512 mb..
had same problems after installing win 7 ultimate rtm..
1) if u change ur graphic drivers
back to vista version.. (not for win 7.. means no latest)
everything is fine..
2) weird bug with win 7 or graphic card cause change in video renderer setting after installation (i use kmplayer, weirdly vmr 9 (windowed) becomes the renderer) and it cause pixalation..
wouldnt recommend haali cause its not compitible with win aero and theme changes to basic , which takes out the charm of aero..
PS I THINK its graphic card (nvidia) and win 7 compatibility issue..
so change video renderer to vmr9 (HQ), or evr or evr (c/a)..