System settings droping out on reseting computer

Cant install Sophos updates
I am running XP with Sophos anti-virus Cant install updates I get this message c:\WINDOWS\SYSTEM32\AUTOEXEC.NT.The systemfile is not suitable for running
MS-DOS and Microsoft Windows applications.Choose “Close” to terminate the
application

The reply From Microsoft:
http://support.microsoft.com/?kbid=324767

Exaplains how to rerun setup from CD and extract the right files. solved the problem, but when I switch off computer and restart, the fix drops back out,I therefore, then have to follow the same procedures.
How can I keep the settings
Thanks

Ron