If it's the 16-bit Sub-system error, there is some spyware out there that deletes the autoexec.nt file.
You can replace the file manually or, my preference, download and run
XP Fix.exe (second one down on that page).
Then use Spybot, Ad-Aware and/or Windows AntiSpyware to remove spyware from your system. Particularly stubborn malware may require scanning with all three.