Data Execution Prevention - virus?

Hi lads,
I keep getting "data execution prevention - to help protect your computer, windows has closed this program" come up randomly on my PC

It comes up when I open Firefox, open My computer, browse files, loads games etc. and sometimes freezes, with the only action resolving it, being to close "drwatsn.exe" i think its called (the dr watson post mortem debugger)

I have run a full spyware scan with AVG spyware, and Spybot Search and Destroy, run a full Virus scan with AVG, and done a registry check and clean with Ccleaner and Computer Tools registry checker I think it was called

I have also run a full defrag on all my hard drives and the problem still persists
I have tried restorying my computer back to a time when this didnt happen and this also didnt work - it went back to that time, but the problem still occurs

Any ideas, as it apparently is there to prevent problems from viruses and other security threats