Probably you have XP and the file you listed is corrupt.
Here are some of the possible fix's"
This installaton program will reinstall the missing or corrupt Windows XP system files
command.com, autoexec.nt and config.nt.
Manually or if you dont have XP
Its the same if you have windows 2000 also:
Operating System: Windows 2000 Professional with service pack 4.
Background: Wanting to upgrade to the latest version of AVG, I decided to first uninstall the old version. When running the AVG uninstall procedure I
received the error message:
C:\WINNT\System32\Autoexec.nt The system file is not suitable for running MS-DOS and Microsoft Windows applications. Choose 'Close' to terminate
Resolution Steps: Apparently this error message, or a similar one mentioning config.nt, can occur when one of the following files is missing or
The erorr can occur when installing or uninstalling a program.
In my case Autoexec.nt was missing from the C:\WINNT\System32 directory. Looking in the C:\WINNT\Repair directory I found that Windows had
automatically made a backup copy of Autoexec.nt, so I just copied this into C:\WINNT\System32 and the problem was fixed.
Note: The C:\WINNT directory on my computer may be C:\WINDOWS on other systems. Substitute as necessary when following the above instructions.
Note: If you can't find a copy of the missing file in your Repair directory, use your original Windows OS disk to get the missing files. At the
command prompt type the following commands:
expand e:\i386\config.nt_ c:\windows\system32\config.nt
expand e:\i386\autoexec.nt_ c:\windows\system32\autoexec.nt
expand e:\i386\command.co_ c:\windows\system32\command.com
Ref:win2k fix autoexec.nt error
If I remember right, this can also be caused by Symantec-Norton Antivirus.
If the above does not help, and you are desperate to install an application, you may be able to stop the service, then install your app, then restart