Error: "The Win16 subsystem cannot enter Protected Mode..."

> I believe I must have deleted a DLL or something
> important when I recently uninstalled a bunch of
> applications.
>
> When I try and install software now I receive the error:
>
> "The Win16 subsystem cannot enter Protected Mode,
> DOSX.EXE must be in your AUTOEXEC.NT and present in your
> PATH."
>
> This is on Windows XP Pro, I am running as administrator
> and I have tried a million things.
>
> Does anyone know what is going on here and have any
> advice (other than a complete reinstall of Windows)?
>
> Is there a way to reapir missing Windows files without
> breaking all my currently installed apps?
>
> Thanks.
>
> Skip