Soundcard and PA woes

[self_titled]

Hi guys, hoping someone here could help me. I bought a PA system for my band, however we can't practice for a while, so I decided to rig it up to my PC for general use. It appeared to work fine and in stereo, however after a restart I appear to get mono sound and a constant "Autoplay" popup. This popup seems to work in the background and slows my whole PC down, rendering Media Center and WMP useless. Any ideas for correcting this? I messed with my soundcard settings, but to no effect.

Thanks in advance.