I appear to have a virus that virus/malware software can't detect

Hi. I have had ongoing problems with my Acer netbook for a long time now. A few months ago, I ran three different antivirus programs and only SuperAnti-Spyware was able to detect a virus, a trojan. But a few months later, my regular anti-virus, Avast, stopped working and my computer won't let me download it again, nor will it let me download any other antivirus or firewall program. So something is preventing it from working. I also can't download the new version of iTunes, flash or other program updates i've tried. I always get errors. Aside from this, the main indication I have that I have some kind of virus is that the sound on my computer is messed up. You can hear it even in the startup sounds. There is always some kind of delay and crackling sound in part of the sound that is supposed to be played. (I hope this makes sense...it's hard to describe.) Last night I ran ComboFIx and came here to seek help understanding the results and what to do next. I saw that I needed to follow the instructions here, so I did so, and have a DDS file I'm attaching. I can also attach my ComboFix file. Please note the DDS file was created AFTER I ran ComboFix. Please also note, while Avast won't open and I have no icon showing it working on my computer, ComboFix kept telling me Avast was working and that I had to disable it. But I could not disable it since I am unable to open it at all. Thanks SO much, in advance, for your help!

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