In laymans (or laywomans) terms the autoexec.NT file tells your computer how to deal with DOS.
It seems that yours has become corrupted. You can edit this file yourself in Notepad buit for a start I woulkd use System File Checker and this may fix the problem.
Put your XP CD in the drive.
Then do Start>Run and type cmd. then OK
At the Window type sfc /scannow if that doesn't work type scannow /sfc
( i'm on a foreign machine and I can't remember which way round the command is)This will repair any corrupt system file.