>Cetin,
>The program does 'set default to &tdir' where tdir is the path to the DBC and DBF files. In both cases the program is started with something like 'myexe.exe c:\vcars'. The program then does the set default. It seems to me that in this case sys(5) should always return 'C:' but obviously it is not and I dont know why.

I'd use :
set default to (tdir)
instead of macro substitution. If you're sure macro subs. is not a problem there I don't have an idea why sys(5) would return wrong info. It always worked right for me.
Cetin