Cannot run DOS Programs

All of a sudden, I cannot run any DOS based programs. The only thing I recently did was install Norton Firewall. Cannot imagine that did anything. Does anybody have any ideas what I can look at to correct situation.
The error message I get is "C;?Windows/System32/Autoexec.nt. The system file is not suitable for running MS-DOS and Microsoft Windows applications."
I have the compatability box checked to run in Win95, Win98, and NO compatability. Same problem

 

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