Originally posted here by Wazz
You can also boot with a boot floppy and at the A: prompt type "sys c:\", this will transfer the system files and overwrite any corrupt ones. This has been known to save my @$$. Happy Hunting....
I need to second that sys c:\ information. It's right on the money. If this is a recurring problem, you might want to make that the virus protection is installed and functional. I seem to recall a lot of old time viruses targeting the MBR and blowing it all to hell - leaving you with that "OS not found" message.

Good Luck!