The Blue Screen Of Death (BSOD) can tell you what Windows thinks the problem is. There should be an error message above the text you mentioned seeing. Generally the paragraph text tells you that if you changed any software or hardware lately, those said changes can be the culprit.
If you want to try to get the computer to stop rebooting at the BSOD so you can read it, use the advanced boot menu. To get to the advanced menu I speak of, repeatedly press F8 after POST (if you can get that far). At the menu, use the down arrow to highlight "Reboot after boot failure" (or something like that), and then press enter to stop the reboots. Your computer will restart again, but when it tries to load Windows and fails, it should stop at the BSOD.
I understand you said that this was a previously working system, and since you didn't mention any changes, i'm assuming you didn't. That being said, I concur with aford10 and the damage hard drive theory. Have you noticed the computer being slower lately (when it was loading Windows)? Have you recently scanned your system for viruses or spyware?