I have a GUI program for MATLAB that won't run on a mac. Can I use virtual PC or something to run it on my mac or would I have to reinstall MATLAB to do that?
In practice, MATLAB's GUI stuff isn't very portable. You'd be better off running it in MATLAB on a virtual machine.
You mentioned Virtual PC. Is yours a PPC-based Mac? If not, you might want to try the free Virtualbox (http://www.virtualbox.org/). I don't think the PPC Virtual PC is being maintained any more.
Having done development on both the MAC and PC, I can say that I strongly prefer using the PC side. While it 'works' on the MAC (2008a), I have noticed when Matlab launches any GUI's (and even worse when using GUIDE) they run much slower. It was slow enough that it made it appear that it did not accept mouse clicks at times. I'm fairly certain that this stems from running Matlab through the X11 port.
Because of these issues I have switched to running Matlab on Parallels Desktop on my 2008 Mac Book Pro and have Zero issues. However, getting to what I believe to be your question, I did have to re-install Matlab on the virtual machine.
The only way around your problem that I can think of is if you had access to the Matlab compiler allowing you to turn your GUI into an executable. This method would allow you to run it on (nearly) any Windows machine and not have to re-install Matlab.
Hope this helps,
Adam