As almut noted, there need to be some
environment variables set for the cl in VC++6 to work properly.

You may accomplish this by invoking (on a standard install, YMMV)
C:\Microsoft Visual Studio\VC98\Bin\vcvars32.bat
as noted in "vcvars invoked?" or by including
the PATH, INCLUDE, LIB, MSDevDir and MSVCDir "by hand".

If the environment variables are set properly, all should be fine.
(This is normally part of the MSVC install process, did you
deliberately not register the environment variables?)

For a first check, open a command prompt and
type "set" (and post the results here)

Regards

mwa