Since getting my computer back from the shop (I basically had it rebuilt with a new processor/motherboard, new drives, etc.) I've noticed that when Windows XP boots, it plays two different music themes at the same time! It plays the "classic" Windows XP theme along with another unknown piece. What's going on here and is there a simple way to fix it?

You could try reverting to the default sound scheme. Go to Control Panel -> Sound and Audio Devices -> Sounds tab -> select the Windows Default sound scheme in the dropdown box. I hope this works, my guess in that there's some other app running at startup that has its own intro sound, or someone added more sounds to your Windows.