If you try to start or install an MS-DOS-based or a 16-bit Windows-based program on your Windows XP-based computer, you may receive an error message that is similar to one of the following:
16-bit MS-DOS Subsystem path to the program that you are trying to start or install C:\Windows\System32\config.nt The system file is not suitable for running MS-DOS and Microsoft Windows applications. Choose 'Close' to terminate the application.
You may encounter to other similar error messages, but with different file names config.nt and Autoexec.nt. This issue may occur if one or more of the following files are missing or damaged:
To restore these files, download the above three files from here. Unzip and copy the missing file(s) to Windows\System32 folder. For more information, see article Error message when you install or start an MS-DOS or 16-bit Windows-based program
This issue which is showing-up lately, seems to be caused by a Trojan (may be TrojanDownloader.Win32.Dia.a) which seems to any of the three files indicated above. Reference: See this thread for more information.
Cleanup the Windows startup applications. Reboot and then run a full system virus scan with updated definitions. If you're unable to run anti-virus program locally, use the Online virus scan services listed in this page. Re-create the three files mentioned above.
Run a scan from Safe Mode with fully updated anti-virus reference files. Additionally, scan with Ad-aware, SpyBot and CWShredder. Update Ad-aware, SpyBot while in Normal
mode . Restart Windows in Safe Mode to successfully scan and remove ad-ware, trojan etc.
A Downloader Trojan (TrojanDownloader.Win32.Dia.a)placed a program called Winad Client on my machine. This is the program that has been removing autoexec.nt and messing with config.nt & command.com.
Last week I replace all those files but after a reboot this weekend the 16 Bit issue came back which I did not discover until this morning, only hours after Norton Antivirus found the Trojan.
Norton AV 2004 could not remove the Trojan Nor could AdAware SE locate it. But PestPatrol (http://www.pestpatrol.com/) found the Trojan and removed it.
After some web searches on this Trojan, I discovered the installed "Windupdater" program on machine is called Winad Client and it was easily removed via Control Panel/Ad-Remove Programs (how silly). Ironically after the program is removed your browser is forwarded to a Windupdater URL promoting Spyware removal.
Further scans show no more Downloader Trojan and the 3 Windows System Files are staying intact.
I would like to add that in my opinion this issue is not an Windows XP SP2 issue. As far as I can tell from the various post, for most people it's just a timing coincidence around their recent install/up of SP2 and the proliferation of this Trojan Furthermore, from my part, I am looking further into whether this Trojan arrived via my installation of the CorelDraw 12 SP1 with it's notoriously invasive & unremovable Update Manager program by InstallShield, since the timing is more correct for my issue and the fact that the Trojan ends up promoting various Warez Utilities.
Take a look at this article for how to fix it:
fixes it. However a reboot will reestablish the problem.
Try this. Fix the file(s) again, and then do a clean boot in Windows XP
If the problem then is gone, you could add more and more (booting in between each change) to the startup list to see if you can find out what causes this.
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