I have a similar system running Vista x64 and with the 8500 GT with the same issues. Freezing when launching computer games, plugging usb external hdd.
I've read forums on the net, it could be due to the Nvidia drivers. A report was out months ago suggesting Nvidia was responsible for 70% of the Vista crashes. I have updated the nVidia drivers to the latest, atm 177.19 and also flash the bios to the latest.
When playing games or firing up some apps, it freezes and locks. Only way to let the rig come good, is to do a cold reboot.
Ongoing issue. Could be a dud 8500 Gt. Regret buying it. Should have got a 8600 GT for an extra $20 or wait long enuff for the ATI 3xxx series cards