Two things you might try.
Boot with your current OS boot disk. Use the FDISK /MBR command to overwrite the Master Boot Record and update it to current OS, this will also eliminate any virus' or corrupted files.
I believe there is a command to overwrite the system files and update those. I could not find it but I believe this will work.
Go to the C: prompt (still using boot disk) and type [del] [file name]This will erase the old system files. Next type [copy][A:file name] This will replace them with the files on the boot disk. The three files you want to replace are- IO.SYS - MSDOS.SYS - and COMMAND.COM For IBM it is IBMBIO.COM- IBMDOS.COM- and COMMAND.COM Use at your own risk and someone correct me if I'm wrong.
I know from the FORMAT command (this would erase the disk) you would use FORMAT C: /S to copy system commands after formatting.
Good Luck!

[This message has been edited by DR (edited October 04, 2000).]