Hmm... (it'd be helpfull to know what OS you are using)
It could be that the boot.ini file is missing.
(assuming that there is a boot.ini file (NT based systems))
The best way to get a boot.ini file would be to either use a dos boot-disk which you can get from www.bootdisk.com
where you can boot into DOS to find the files.
If you use windows NT, 2000 or XP then there is a chance you have an NTFS filesystem, in this case you'll find that a DOS boot disk won't be able to 'see' the disk. In this situation you could use a Linux boot disk, (I recomend Toms Root Boot
Linux on a floppy). This will let you access an NTFS partition.
Your boot.ini file is the file that points to the opperating systems on yuor hard disk.
and should lok something like...
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
multi(0)disk(0)rdisk(0)partition(3)\WINNT="Microsoft Windows 2000 Advanced Server" /fastdetect
You could repair this using the recovery console on the installation cd (as LK said earlier)
If you have a pre-Nt system.
Windows 95/98/98SE/ME then it could be that your boot secotor has been corrupted. in this case you should use the DOS boot sidk from bootdisk.com, noce you have DOS loaded on your sysem you should type
This will erase and re-write your bot record and hopefully sort out your problems.