Well to be honest this is a problem you bring on yourself. Zune Markeplace allows you to put your collection on up to 3 PCs (if I remember correctly). Honestly, how many times do you need to do this? If anything I would think you would keep your primary PC out of the testing environment because too many things can go wrong.
If you haven't already done it, you should go into settings in Marketplace and be sure that you only have a certain amount of PCs on your account. One thing you might be able to do everthing time you set-up a new PC is to be sure to Name the PC a name thats already on your list of allowable PCs (of course this should not be a PC thats actually in use). I would hope that you are smart enough to keep your music on an external drive because if you are wiping out your music every time you test a new OS (like some other poor sap did) you will eventually lose the ability to download those tracks.
But if this really is a problem for you the easiest thing to do is keep your primary PC out of the testing envrionment all-together.
Welcome to the Dawn!
NPGMBR