"This sytem file is not suitable for running MS-DOS and Microsoft windows application

Try renaming the aforementioned file to autoexec.nt.bak. If the problem goes away, you can likely delete the file.

I would be interested to see what that file contains.

Wtf, there is no AUTOEXEC.NT
Mabey some virus deleted it. Time to find my WinXP CD..

You don't need an autoexec.nt. I was interested to know, because it's not normally used.

Hm... aparently its a SP2 problem, looked it up on google and found a patch

No, it's not an SP2 problem. It's common for some spyware and adware to tamper with this file. Also, you DO need this file as it is a part of the 16-bit subsystem neccessary for running DOS and some very old (16-bit) Windows applications.

You can find a backup copy in C:\WINDOWS\Repair

I don't have a copy on my system.

Hmph. If you need DOS compatability, forget using Windows - Microsoft needs to be encouraged to just walk away from it. Instead, use something like Dosbox. It's safer and I haven't found a DOS game yet it doesn't do (though you need a VERY powerful machine). This way, if it crashes, you just close the window... it's in its own box and can't affect the rest of the OS.

Also, scan your system with Adaware and Spykiller - something may have come in and is screwing with you.