Windows 7 - Problem with mapped drive

I run a small network in my business. The software I use is perfectly adapted to my use, but it is written in DOS Basic. This limits my access to network devices.
Until now using windows XP, I solved this problem by choosing one hard drive (C) as my server drive and mapping a (Z) virtual drive in every computer to that (C) drive.
I can access and update info in the server (C) drive from any computer, once I have shared and established permission levels. I share the root of the (C) drive, so everything is accessible.
Now I try this with Windows 7 and I cannot access the server (C) drive from the virtual (Z) drive from my programs, although I can see the (C) drive when I open the (Z) drive.
I have tried every possible level of permission settings, and still no luck. My question is this ...am I missing something here, or is it just impossible to do this in Windows 7?

I appreciate any and all advice.

Thank You

Rick