If I'm right, I think this is a virus called 'Black Internet', which my PC was infected with 2 weeks back. This causes extra iexplorer.exe processes, audio adverts to be played, and sometimes issues with the volume (for example, my 'Wave' bar in the volume options would mute randomly, like demonix referred to). The best way to find out for sure is to check your temporary internet files, looking for a 'loader.exe' and 'smss.exe'.
If it turns out you do have this, to get rid of it is fairly awkward, as it buries itself in the master boot record, so you have to fix that, after which a Malwarebytes Anti-Malware scan
should find and get rid of the rest of it.
I'm not the best at explaining these things, but there are a few walkthroughs to remove Black Internet, such as these:
http://www.bleepingcomputer.com/forums/topic322695.html
http://forums.steampowered.com/forums/showthread.php?t=1360060
http://forums.steampowered.com/forums/showthread.php?t=1358896
The Steampowered ones explain the method I used, and I am on XP SP3 if it matters.
Incidentally, I fixed my infection of it because my PC wouldn't boot (hung on 'Verifying DMI pool data'), which was fixed with 'fixmbr'. After it came back on I ran scans through habit and removed the infected loader.exe and other files. It wasn't until the next day that I discovered what I'd had.