How do I fix my computer from turning black whenever i change resolution, switch user, etc?

First I'll state the problem and ask the question at the end. Whenever I change resolution, switch user, log off, screen saver turns on, or have the monitor turn off after one hour, my screen will turn black and stay that way. Moving the mouse and pressing buttons won't work. My graphics card is an Evga Geforce gtx 260 (takes minimum of a 500 watt supply and I have a Rosewill 600 watt supply) upgraded from an Evga geforce 8800 gts (that card worked just fine). I have the latest driver from Nvidia and even tried the driver with the disc that it came with. The only game I have tried to run thus far is Mirror's Edge. Whenever the menu tries to load, the game crashes. It worked fine with the other card. My question is, what is the problem, and if one knows what the problem is, how would I go about fixing it?



Relevant Computer specs:

Originally a stock computer from I think '06 (Model: Gateway gt5654)

Windows Vista 32-bit (stock)

Rosewill 600 watt power supply (after market)

Evga geforce gtx 260 (after market)

AMD Athlon 64 x2 Dual Core 5000+ 2.6 GHz (stock)

3 gigs of pc 5300 ram (stock)

1 PCI-E slot (stock . . . somewhat obvious)

Motherboard is, as far as I know, just plain %26quot;Gateway.%26quot; I typed in the serial number and every other relevant number I could find on the motherboard. I just found it to be %26quot;Gateway.%26quot;How do I fix my computer from turning black whenever i change resolution, switch user, etc?
Do you have your discs set to turn off at any point in time? If so, disable this option.