Hi professornayr,
When you connect to a second PC you should have a second partnership established on the device (Moto Q) - is this the case? Run ActiveSync on the Moto and use the right hand soft key for the Menu and look in Options. You should have two devices listed - the Vista PC and the XP PC.
If this isn't the case, you haven't got a fully functioning partnership with one of the devices....
As a way of getting things to connect, establish the partnership but untick ALL boxes for items to sync. This will establish a connection to the device. Once this is done, you can go back into WMDC (Vista) or AS4.5 (XP) and tick individual boxes for items to sync. I tend to do them individually to identify if any single one is causing an issue.
Also, just for info more than anything, you can only have e-mail sync'd with one partnership.....
Let us know if you are still having trouble.....