My new PC I built at christmas is freezing at random times sometimes the screen turns purple or green, and I have to do a hard shutdown the screen going purple or green could be my 8800GT or my monitor.
There has been reports of 8800GTs having crippled drivers, which may potentially causes lock ups. I think you should update to the latest driver from Nvidia's website. If that still doesn't work, try using one of the beta drivers.
One of my friends also had lock up problems. The problem exacerbated, and in the end, he switched to CF 3870s.