audio ads help!

hi, for some reason audio ads keep appearing on my comp. when i alt+ctrl+delete, nothing shows in the program list, and the ads change everytime i hear them. i've run ad aware and spybot, and nothing can fix it. is there any way i can get rid of these annoying without having to reformat??? the only thing that i've downloaded recently was STEAM, which is a program to play the online game counter-strike on. can anyone add insight? thanks a million.