Unable to Run Windows Normally and cannot invoke Safe Mode

After 2 years, my PC won't boot up. Windows 7 x64. ASUS P8H67-I. System always reports that it needs to repair. I've run Startup Repair perhaps 10 times, and it never finds anything wrong.

SAFE MODE
I can't get into safe mode when booting from disk. F8 is ignored. But I can get into safe mode if I boot from DVD.

I'm able to boot Linux from a repair/troubleshooting distro. At the moment, I'm running something called Mini Windows XP from that DVD, which has an Opera browser. The PC hardware itself seems OK. No disk errors. No memory errors.

CLUE
The only clue that I have is that before this trouble started, I got the little balloon notice that there was an Unknown USB Device. I shut down normally, but since then, the system has not come back up.

QUESTION: is there a way when booting from DVD to delete all USB devices from the configuration?

SYSTEM RESTORE POINTS
I had a half dozen or so system restore points saved, and tried restoring to each one, working my way back in time to one from about 30 days ago. Each time, the system restore succeeded without error. But I'm never able to boot normally.

As soon as the Windows will 'o th' wisps appear, things come to a halt, and the Startup Repair dialog appears.

I've tried booting from DVD but get the same Startup Repair message there as well.

What this turned out to be was the bios had the system drive set to IDE mode when it should be set to AHCI. I have no idea how that BIOS setting change happened. I certainly did not do it. All I did was shut the system down gracefully and tried to turn the system back on.