jjja, You could go to the link below and download "XP_Fix.EXE".
"This installaton program will reinstall the missing or corrupt Windows XP system files command.com, autoexec.nt and config.nt. The absence or corruption of one or more of these files causes a "16 Bit Subsystem" error.
We recommend that you download this file and save it to your desktop or to another location where you can find it. Double click on the file to run it once it's downloaded. If problem reoccurs in the future, simply re-run this program".

http://www.visualtour.com/downloads/


Another fix some have used is to do a search for autoexec.nt You will probably find it in C:\WINDOWS\repair. Copy and paste it into C:\WINDOWS\system32.

The 2 links below explain what may be causing the problem (Malware).

http://www.vtsoftware.co.uk/users/MarkDowsett.htm

http://groups-beta.google.com/group/microsoft.public.windowsxp.general/msg/a448971496ce38b5

Hope this helps,
Tufenuf