From the Symantec website regarding 16-bit MS-DOS subsystem errors:
"According to Microsoft technical support, these errors are linked to the following registry key:
This key has an VDD binary value that points to:
If the VDD binary value is removed, then the 16-bit subsystem returns to normal and is functional.To solve this problem, you need to remove the VDD binary value first, and then update the Symevent files."
"Click Start > Run. The Run dialog box appears.
Type regedit and then click OK. The Registry Editor opens.
Navigate to the key HKEY_LOCAL_MACHINE\SYSTEM
In the right pane, double-click the VDD value. The Edit Binary Value dialog box opens.
Select the contents of the Value data box, and then press Delete. There may be four zeros that cannot be deleted.
Click OK, and then Exit the Registry Editor."