Black screen only

I need some advise on what to do with a computer that after booting only gives me a black screen with a mouse cursor. I locate where the user box should be in middle of the screen and click and I get my desk top appear for about 60 seconds before it goes back to the black screen with only the cursor.
I recently put a 850pro agp in that computer with 7.4 drivers and all went well considering the age of that old pc my son played stalker all last week without a problem until today this happened.
I was able to remain in windows for a while today before the black screen (nothing I did though) and was able to see a couple of things that were wrong. 1: being catalyst control panel would not let me in anymore . 2: the computer is in need of a de-frag.
I un-installed the drivers and did a re-install but upon restarting I got the black screen before I could review anything.
One other thing, this card has as-well as a molex power connector and a smaller yellow 4 pin connector as-well but I have only the molex plugged in is that correct? Would the move from a 9800 pro to this 850 pro cause the psu to give me this black screen as I do have far more powerful psu's laying about the house if I need to change it.
I even have a unused 750 watt OCZ doing nothing in a box.
My 2 main computers use Nvidia cards so I am very rusty with ATi now.

Hi Mark,

You probably have this fixed by now but in case you don't....is the card/system running fine without a driver installed? How are you removing the drivers? It certainly wouldn't hurt to try the bigger PSU but it sounds more like a driver/OS problem to me. How much free disk space does the C:\ Drive have?