Your autoexec.nt has been corrupted, either by a virus or spyware,
user-error, or hard drive corruption.

To fix copy the autoexec.nt from c:\windows\repair to
c:\windows\system32 or copy autoexec.nt from your original xp cd.

If you don't want to bother doing that then download the Windows port
of ROTT: http://home14.inet.tele.dk/Bna-Info/DL_WinROTT.htm

Download WinROTT

This is a known situation.  Jimmy usually posts this solution:
=====
Here's an easy method to solve that error:
http://www.kellys-korner-xp.com/xp_m.htm#16bit

If that doesn't correct the issue, try this manual method:

1. Browse to C:\WINDOWS\repair
   Right-Click and Copy the file named: autoexec.nt *

2. Browse to C:\WINDOWS\System32
   Right-Click in that window to Paste the file
   (Answer yes if prompted to replace existing file)

* Note:  If the file autoexec.nt doesn't exist on your system
right click the link below and select the "Save Target As.."
option to save the file in C:\WINDOWS\System32
http://mvps.org/nibblesnbits/AUTOEXEC.NT