A friend of mine was cleaning out what he thought was cruft from his
c:\Windows\System32 directory when he deleted oembios.dat. His computer failed
to boot after that, and a system restore disk didn’t help. Although he could boot into a command prompt, he couldn’t boot up in safe mode. He fixed the problem
by copying an oembios.dat file from another computer. Read more about this here. The oembios.dat file may be related to Windows Product Activation.