Your cmd.exe file might have been replaced or become corrupt. From Microsoft: (It's talking about a different version of windows so on Windows 7 you would replace Windows\system32\cmd.exe from another Windows 7 computer or disk.
To resolve this issue, replace the Command.com file in the Windows\System32 folder with the same file from another computer that is running Windows NT Workstation 4.0 or Windows NT Server 4.0, and then make sure that there are no Command.com files dated 7/11/95 on the computer. You can also replace the Command.com file on the computer with the same file on the Windows NT installation CD-ROM.
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