Windows won't start up?

Yesterday, My computer stopped functioning properly. I don't know what caused it because everything was working fine until I restarted it. I got a new ATI graphics card and was installing drivers, but it was still working fine. I was playing games and viewing videos just fine with the new gpu.

Anyways, everytime I start my PC it goes fine up to the "windows is starting up" screen, then my computer restarts and repeats over and over again.

I used F8, and tried every possible option, but my computer wont go into safemode or anything. (It just keeps restarting) Also, my system restore disk gives me an error whenever I try to run it. (Says my drive is not a cd drive?)

I'm assuming the ONLY way to fix it is from the BIOS, or some kind of USB Reboot thing. I tried messing around with the BIOS already, and nothing worked. (Optimal settings didn't help)

Anyone know of a solution for this problem? (Other than getting a new HardDrive) Thank You!

No, I did not upgrade my PSU. I'm sure it's not a watt problem either considering it's a 500watt psu and the gpu only asks for 300.

My graphics card is an ATI HD2600 Pro. (PCIe)

I think Aford10 is on the right track. That 500W power supply might only really give out 350-400W, depending on the quality. If I remember correctly, 2600XT does require 1 six-pin PCIe power connector, and something like 18Amps to run. A crappy 500W power supply might not provide enough amperage to run the entire machine with the 2600XT in there. (I had this experience with a 6800GT).

The other issue to consider would be heat. If your CPU is overheating and it is at least a P4 or Athlon 64, the CPU will first clock down and then shut down completely in an overheating situation. Take a look and make sure all of you cooling fans, especially those on the CPU and 2600XT are working and clean. If this is the case, try another power supply that supplies at least 30Amps.