You are in the usual position of the seller blaming the software and so on.
What I would try first is Windows update to see if there is any updates. Microsoft are notorious for releasing products which are then tried and tested by the public, so any issues are then reported back and they then try to sort out the bugs with patches etc (XP has had a massive update with Service pack 2, virtually rewritten I believe and the other day I downloaded 32 updates since service pack 2!).
Also right click My Computer>properties>hardware>device manager and check everything looks ok in Device Manager. If it is, then my guess is that it is most likely the software ie, WMC. It is very hard to trouble shoot this sort of thing, short of ripping each component out and then trying others. As I say, try updates first and make sure you have just WMC and little else installed first if possible. If it has anti virus installed by default, also try disabling this for a short while to see if this is causing it.
Regards,
Tony H
PS: Once you have done all of the above, if it still does not work...in my opinion you have every right to take it back! However, dont go telling them too much like you tried updates etc or they might try and blame that!