I would have to say its either your Motherboard, video card, or power supply then.
My personal opinion would be to borrow a friends video card, and power supply. Otherwise it could be your motherboard. I dont want you to shell out extra money if the problem is still gonna happen.
Before you do that? is that board a nForce 4 Ultra? or a nForce 3 250 GB? or regular nForce 4?
With a computer, its magic to get it actually working. A lot of things could go wrong.
It could be either software or problem:
I think if its software problem:
Then Most likely its a driver problem: I dont know about your board, but I have a Msi board, check my rig, and the instruction manuals explicitly states to NOT install CERTAIN nForce, or Forceware or nvidia drivers. Check the instruction booklet if it says that.
I would personally uninstall any sort of nvidia or other drivers you would not need, such as the nforce firewall or nforce IDE drivers.
Another thing is there might be an IRQ conflict with the Video Card. The Video card is sharing the same IRQ with another device, check the BIOS settings and perhaps assign the video card a different non used IRQ number.
If its hardware:
It could either be video card, whats a Leadtek PX6600TD GeForce 256mb? is that agp?, i thought 6600 models come in GT , regular and 6600 LE.
Faulty motherboard
or Faulty PSU.
It could be ram like people stated, but I doubt it, I think theres a conflict with your video card and your motherboard. Ask Leadtek or go to their website to see if thers a bios flash for your video card.
Good luck! and report back if problems