Software Conflict

When I try to install some software I sometimes get a box come with the following message in it:

16bit Windows Subsystem ? C:\Windows\System32\Autoexec.NT

The system file is not suitable for running MS-DOS and Microsoft Windows Application Choose ?Close? to terminate the application.

I am then given a choice of 2 boxes with Close in one and terminate in the other.

I have tried to rectify this manually but i got as far as this

On the File menu, click Save As.
In the File Name box, type Autoexec.nt, and then click Save. Close the Autoexec.nt file

When i have typed the text and in the solution above, i get a text file and not a .nt file. How do i solve this.

Can anyone help as this is preventing me loding some of my software.