I have been facing this annoying error for quite some days now. It happened for almost all the commands I try to perform including those general command: cmd, regedit, or even ping. It's really bugging me and I've been searching for the fix but apparently it have been circulated that it was a virus/worm.
With such info on hand, I went and try different type of anti virus software to make sure the virus is eliminated. There was indeed few viruses detected but the problem wasn't solved. The command that I wanted to use still not accessible.
I also did a "sfc /scannow" command to perform a verification of all the windows driver are intact but to no avail. As I just found out another way as quoted below from the source;
MS-DOS or 16-bit Windows-based program Error Message
This installation 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.
XP_Fix.EXE by TRF Systems, Inc.
Running Audit on my computer showed that a program called
WINDUPDATE.EXE (not WINUPDATE.EXE) was deleting my Autoexec.nt file on boot
up. If I replaced the file it would delete it again. Go to \Program
Files\WINDUPDATE folder and delete it!
Go to your registry and delete anything named WINDUPDATE. Put the Autoexec.nt
file back into your \system32 folder and all is well! This was definately the
problem here and I bet most everyone has the same nasty bugger, WINDUPDATE.EXE
on their infected computer.
Hope this would fix the problem now. Will restart my pc and update the status.