Is the Windows mixer set to max?

Start > control panel > Sounds and Audio Devices > open mixer

I "upgraded" to WMP11 from 10 and didn't notice any volume degradation

Play a track in it, and make sure you are on the NOW PLAYING screen. Open the enhancements, and go to the "cross fading and auto volume levelling" menu, and make sure both are turned off - that WILL make your music quieter if turned on.....