Well, I am using some software designed for Windows 95 on my XP workstation. But, they are more office-type apps, not professional entertainment apps.
The biggest issue you really face is the Com port, which can be most likely be addressed via a USB adaptor.
I have to get a USB adaptor for this purpose myself, but my application is so I can have support for my analog audio console firmware. I also am planning a purchase of a digital console within 6 months.
My advise: Since you claim to have some experience with lighting, I say just tell them to spend the money or waste time with lights not working as well as they should. The place is sounding a bit run down. Owners often don't understand this stuff other than "It works or it doesn't work". I have nothing against non-DMX FX and dimmer and switching packs, but I think you need to do what you've been doing and that is to give that place an overhaul.