Download this free program NTFS4dOS from :
http://www.datapol.de/dpe/freeware/Install on a working pc and let the wizard make a floppy.
Boot the dodgy pc using that floppy, when its booted and asked if you are using for private use,
type YES
Then type the instructions as follows :
At the A: prompt type C: (hit enter)
CD windows\repair (hit enter)
copy autoexec.nt c:\windows\system32 (hit enter)
That should find the correct file and replace it if it is missing or repair it if its damaged or corrupt in the system 32 folder.
If it ask if you want to overwrite say YES.
I hope that works as Scrumpy Jack has been helping me to work through it and I had it working on a spare pc up to the overwrite part, so it should be ok