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This error message pops up in Microsoft operating systems when the boot device cannot find the COMMAND.COM file or the file is damaged (perhaps by a media error or a virus).

There are several ways to correct this error. If you have COMMAND.COM located somewhere other than your root directory on your C: or A: drive, you need to note the location in the CONFIG.SYS file using a text editor on another machine or after booting with a different A: diskette that contains a valid COMMAND.COM. The other possibility is to boot with a write-protected bootable diskette that has been formatted with the same version of COMMAND.COM as your hard drive, then just copy the file over the version on your hard drive.