I've been driven to distraction over the last few days - Media Centre (64 bit Win 7) has lost its music and no amount of rebuilding libraries has helped.
Finally had to ask for help and cracked the problem by artificially adding then removing items from the library. Along the way I discovered that the performance of music in WMC is massively improved by stopping Windows Media Player from accessing the internet for album covers & info (set it manually and be done with it).
I also found that the default WMP on Win 7 64 bit is the 32 bit version (so you can change the short cut to point at the 64 bit folder rather than 32 bit, no problems there).
Also documented in the posts I made on The Green Button (title above is the link) are some checks to ensure folders are set correctly in the registry - they probably aren't if you've made drive / path changes.