A quick Google search revealed that this is a  common problem.  It 
appears that XP sp2 can overwrite the Autoexec.nt file.  You can 
either copy it back from your XP disk, or even find it online.

Tony Mayer




>I have seen this in an enterprise environment where we use a software
>distribution system. I suspect a Microsoft patch or update could have
>overwritten or damaged the file. But like you said this would seem to be
>a critical system file and without you can not perform any software
>installations without this particular file.
>
>I am also looking forward to seeing an answer for this problem if anyone
>else has an idea or opinion.
>
>-----Original Message-----
>From: Personal Computer Hardware discussion List
>[mailto:[log in to unmask]] On Behalf Of Hugh Vandervoort
>Sent: Tuesday, April 11, 2006 2:40 PM
>To: [log in to unmask]
>Subject: [PCBUILD] Autoexec.nt missing
>
>I was trying to install AVG on a Dell 4500S today and got a 16 Bit
>Subsystem error. I was also unable to turn on XP's firewall. I
>subsequently discovered Autoexec.NT was missing.
>This is a reasonably well maintained machine with a timid user who only
>uses it for email. Her 20-something daughter occasionally uses it for
>email also.
>I was able to extract the file and solve the problem, but I'm wondering
>how this important file could have gone missing. The logs show no signs
>of viruses or Spyware.
>Any thoughts would be appreciated.
>
>                          PCBUILD's List Owners:
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>                        Drew Dunn<[log in to unmask]>
>
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