Audio Channel Configuration Question (I think). Why? WHY?

[Superman]

Ok, so I usually use my PC when I practice playing my guitar. I use a hardware component called a USB Fast Track that lets me plug the guitar directly into the PC without using a separate amp. My setup looks something like this:



I can then use a software program to add effects like echo or distortion. Now, here's the problem; whenever I open another media file that has audio (an MP3 or flash video I want to play along with, for example), my guitar's audio immediately changes back to the default sound. In other words, I can't use any guitar audio effects when another audio file is playing. I can still play and hear the guitar, along with the other music file in my headphones, but I can't use any of the guitar's effects. It immediately changes back until I stop the other file.

Anyone know why this would happen? My PC's hardware?