What is AC3 Filter

Sorry 2 sound like a noob but…

I seen some programs appearin in my folders…which i haven’t downloaded :-
Latest is AC3-Filter which seem as a soundtech-thing.

Also other which i have deleted…
I have winxp, i use avast antivirus, zonealarm, spyboot, adaware and microsoft antispyware.

Is this enough 4 protection against hidden download\installment of programs that i am unaware of and potentially could damage my machine or attack privacy?

AC3 Filter is a software decoder for Dolby Digital encoded sound.