Data card modem not recognised

We use a Sprint data card modem that plugs in to a USB port. I move it back and forth between my laptop & desktop. Our old modem physically broke so we got a replacement one. It worked okay for a few days and then the computer would quit recognising it while I was online. This was only on the desktop, it worked on the laptop okay.  I could reboot the computer and it work again for a little while but eventually quit altogether.  We sent for a new modem which again worked on the laptop but this time the desktop won't recognise it at all.  The USB ports are all okay, they work with the ipods, etc.  I've done many hours of troubleshooting with Sprint but they say it's the computer not their modem that's causing the problem.  The software has all been uninstalled and reinstalled and it claims that the drivers are all the most up to date.

 Any suggestions?

We are using Windows XP