Ah yes, good old Timeout Detection and Recovery. This bug can be triggered by a ton of stuff and as such the solutions vary quite a bit.
What Tog said is a good first (and hopefully last) step. In most cases TDR kicks in when it shouldn't which is why it's technically a bug not a feature. If you've ruled out harware or driver issues, as a last resort you can either keep the theme basic and see if it helps or change the TDR settings.
Since they removed the controls from windows 7 you'll have to manually add them back in so don't try this if you don't know what you're doing. Using regedit enter the following keyvalue
KeyPath : HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\GraphicsDrivers
KeyValue : TdrLevel
ValueType : REG_DWORD
ValueData : TdrLevelOff (0) - Detection disabled
TdrLevelBugcheck (1) - Bug check on detected timeout, for example, no recovery.
TdrLevelRecoverVGA (2) - Recover to VGA (not implemented).
TdrLevelRecover (3) - Recover on timeout. This is the default value.