C:\Windows\System32\Autoexec.NT

I have recently connected my laptop to my library's internet accessible connection.  Since then, upon startup of the computer I get a message from a 16 Bit Windows Subsystem dialogue box stating "C:\Windows\System32\Autoexec.NT is not suitable for running MS-DOS and Microsoft Windows Applications. Choose 'Close' to terminate the application".  I cannot run an older program called X-Tree Gold because the same message comes up and prevents me from opening the program.  System Restore and reinstallation of XP-Professional did not help.  I would appreciate any solution to this problem.  Thank you.