Is there a way to disassemble Pro*C/C++ executable files?
In general there should be disassemblers available for executables, regardless how they have been created (gcc, proC, handwritten, etc.) but decompiling an optimized binary most probably leads to unreadable or source.
Also, Pro C/C++ is not directly a compiler but outputs C/C++ code which then in turn is compiled by a platform native compiler (gcc, xlc, vc++, etc.).
Furthermore the generated code is often not directly compilable again without lots of manual corrections.
If you still want to try your luck, have a look at this list of x86 disassemblers for a start.