Hello and welcome to Driverheaven!
Sorry for the late reply to this thread. It's been well overdue
Is this the only thing that has been changed to the computer in terms of the upgrade? Or did you get this video card along with many other components? If you got several other components, then you want to test those out as well since it is not just the video card that is new.
When is it that the computer freezes up? does it happen when playing games or does it happen randomly? if it happens randomly then it could also be a system memory issue. It could also be an issue with the motherboard itself, likely the northbridge controller is faulty. Again, this is all assuming that you have several new components.
If you could please provide us with the rest of the system specifications, we would be able to further help you out here.
video card: 8600GTS OC
motherboard:
RAM (size/type/speed):
CPU:
PSU:
OS:
those are the most important to know.