Maybe Windows Update broke it!
I had this same problem with my Vista Ultimate media PC a while back. In my case, a Windows Update was the culprit, and it actually left my media PC at home unusable for 5 months until I finally got around to spending a few hours messing with it. I had to boot into safe mode, login as an administrator, and start uninstalling recent Windows Updates. I was going on a hunch that the latest Vista Service Pack broke my machine, but after I uninstalled a few of the post-service pack hotfixes and rebooted, the problem was fixed.
Fortunately, I only had to go back about 6 updates before the computer stopped rebooting on startup. After that, my computer booted up happily, and I was able to install Windows Updates one-by-one until I was all up-to-date. (My guess is that the buggy Windows hotfix was replaced with a later hotfix that didn't mess up my computer. You might have to identify which update messed your computer up, and choose not to install it.) Since then, I haven't had a problem.
Or possibly Malware
Your computer could also be infected with some type of malware. If that's the case, repairing Windows will not help. I'd suggest trying to remove the malware first.
I've observed that some malware seems to go temporarily dormant if you physically remove the machine from all networks. Assuming you can download files using a different computer and burn them to CD (since we don't trust the infected computer not to infect a USB drive), you can unplug the Ethernet cable and disable wireless networking, if applicable. This might buy you enough time to log into Windows and start fixing things.
I've had a few friends/family members/colleagues who have had the same symptoms, but in their cases a virus or worm was to blame. You can try these resources to help:
Malwarebytes' Anti-Malware - this worked for several computers that my boss owned, when nothing else seemed to work. I highly recommend you try this first to zap any viruses/worms.
SDFix - this has worked for my computer and some family members' computers when our computers were infected even though we were actively running anti-malware programs.
BleepingComputer Security forum - a community of volunteer malware removal experts. They keep on top of the latest threats and can walk you through identifying and removing the malware from your computer if the previous steps did not work.
If it's not malware and it's not a Windows update, then you should continue with other steps and try repairing or reinstalling Windows.