It's funny you should say this, I had the EXACT same problem on my sisters pc. Same file, same message. I just copied over the autoexec.nt file from her backup CD and it worked just fine.
I think at some point, it might get edited/altered on your pc, so find your recovery CD and just copy the one file over. It worked for me, maybe your problem's more advanced.
To be extra careful, backup your current autoexec.nt file first.