My computer has been acting really weird lately. First i was having trouble with games glitching and freezing, then playing any games, even on their lowest graphics settings resulted in either a crash-to-desktop or a full pc crash. Now the problem has spread so that even when not putting the pc under any stress the glitching appears on my desktop. Heres an example of the problem:

It doesn't happen with just CS:S though. It happens with all my games and sometimes when i'm not playing games at all.
Its annoying cos i seem to have tried everything. I've already had to return one 6600GT due to problems so i'm hoping that lightning doesn't strike twice. I have a Zalman VGA fan blowing on to the GPU and Expertool tells me it's running at about 40 - 45'C. My CPU runs at around 45 normally and about 50 when playing games. My mobo is a constant 39'C.
I've tried reapplication of thermal paste and dusted out the case with a can of air. I've fitted two case fans now and that hasn't helped.
I've run antivirus and anti-spyware software and that didn't help either. My drivers are up to date and i only play games on medium settings. The symptoms suggest to me overheating but my computer seems to think the temps are acceptable. I reckon it is my graphics card cos when it crashes the error message says something about the file "nv4_disp.dll" which is my main graphics driver. I really don't know what to do next. Please help.

It doesn't happen with just CS:S though. It happens with all my games and sometimes when i'm not playing games at all.
Its annoying cos i seem to have tried everything. I've already had to return one 6600GT due to problems so i'm hoping that lightning doesn't strike twice. I have a Zalman VGA fan blowing on to the GPU and Expertool tells me it's running at about 40 - 45'C. My CPU runs at around 45 normally and about 50 when playing games. My mobo is a constant 39'C.
I've tried reapplication of thermal paste and dusted out the case with a can of air. I've fitted two case fans now and that hasn't helped.
I've run antivirus and anti-spyware software and that didn't help either. My drivers are up to date and i only play games on medium settings. The symptoms suggest to me overheating but my computer seems to think the temps are acceptable. I reckon it is my graphics card cos when it crashes the error message says something about the file "nv4_disp.dll" which is my main graphics driver. I really don't know what to do next. Please help.