1) The easiest solution is to get a little freeware program at versiontracker.com called "iTunes Associator" which will change creator/type automatically.

2) In the control panel "File Exchange", make sure that mp3 is set for iTunes, not QuickTime or RealPlayer. Some PC users actually use RP for mp3's, so when you get one of those, it really isn't an error. Even if these are set properly, check also the control panel "Internet". Despite all of these precautions, sometimes you still get overridden, in which case, just use Associator.