Put me down for missing or corrupt boot. Also, remove any external storage devices
Also remove ANY non essential USB devices and test. I once had a Dell OptiPlex GX270 (in 2005 or 2006) that did this and it was the printer. More specifically the card reader in the printer!
It didn't do it every-time. I had tested the power supply, check the motherboard for bad caps, ran the quick Dell Hard Drive Daignostics... All passed. I reset that bios with ALT E F B... Pulled the jumper to reset it! Flashed the BIOS...
Then I swapped the whole damn computer with one in another room (and swapped the hard drives)... the problem still stayed in her room! I presumed it was a software problem NOT a hardware problem.
1. Imaged the computer... still a problem. Okay... figured it MIGHT be a bad HDD that tests good.
2. Replaced it from a box without marking the other one BAD (I had like 40 spare drives)... Imaged...
Same problem.
At this point every part was different at this woman's desk! She had a clean/new HDD with a clean image that was working on 1,000's of computers.
I had already looked and saw no externals.
Next, I unplugged everything except a keyboard, mouse, monitor & power...
It worked.
I added one thing back at a time until I found something that failed... Then I added everything else back and traced the cable... An HP Printer.
I had an epiphany that it was the card reader!
I then looked in the BIOS and was able to change something so that the GX270 was not even going to look for USB boot devices... <== That fixed it!
^^^^^^^^^^^^ This problem took about 45min to 1 hour due to the imaging & part swaps. I learned something though!