I recently purchased a XFX Ati Radeon 6450 HD and faced the same problem.... i tried all the possible solution on could find by googling like underclocking the GPU, playing with advance boot options, disabling aero... however none of them solved my problem...
As a final try i went to ATI's official site to mail them regarding this issue where i found a page related to this problem. As per the page there are several reasons for this to happen like insufficient power from supply... improper driver installations etc...
at the start of article they had mentioned about TDR issue faced in windows Vista and 7.
You can read about TDR in detail here
http://msdn.microsoft.com/en-us/windows/hardware/gg487368
After reading the article all i did was
- open REGEDIT
- goto HKLM\System\CurrentControlSet\Control\GraphicsDrivers.
- Add following DWORD "TdrLevel"
- Set the value to "0"
This disables TDR. I dont see a downside in doing this. SO far my system is working fine. I hope both windows and Ati come up with some solution which doesnot require playing with registry