Legacy Application Only Works With Particular Network Switch?

We have a legacy client/server application, an OCR system. The server runs on Windows 2003 R2. We have client machines running happily on Windows XP.We have for a while had problems with getting the clients to run on Windows 7. Some machines wouldn't work, others worked intermittently, one worked consistently. By a process of elimination I discovered that the one working consitently was on a desk where we had put a small 8 port 3Com gigabit switch on the desk to give us more network ports. If I move this switch to any other desk to one of the non-working clients and connect them using the same uplink but going through the desktop switch they start working.The XP clients work fine using any of the network switches. None of the 100+ devices on our network (desktops, laptops, VoIP phones, printers etc) are showing any other signs of network issues, only this OCR application. The application was last updated in 2004, so was not designed for Vista or 7, essentially the vendor is no longer updating it, but say that as far as they are aware it works with Windows 7.why adding a 3Com 3c1670800B switch between the client and the rest of the network should make the application work with Windows 7. Whilst I have a workaround for the problem I'm uneasy that I don't understand the root cause, and if I was to press ahead and move the client machines to Win7, the whole system depends on a single piece of kit that's now discontinued and I only have one of. It all seems pretty counter-intuitive to me that an application should be sensitive to low level network hardware.