If the boot sector got screwed up you can repair the BCD in recovery console. I believe the command is bcdedit.
First rule out viruses and failed software installation and updates. I find that a lot of times when windows randomly fails to start it is because of a failing disk or bad sectors that happened to affect a needed system file. Take the disk out of the machine and put it in another machine. Copy off all of their important data. Then, you can put it back in and run chkdsk /r in recovery console to find and attempt to repair any bad sectors. You can also run checkdisk with the disk hooked up to another machine. I find that many times this will repair the boot sector by reallocating it to an undamaged area. If this doesn't work, then I usually just do a nuke and pave because I can't afford to spend any more time screwing around with the computer.
But, before I reinstall, I test the disk out with a tool like HDDscan to check for bad sectors. No sense in reinstalling on a bad drive....
