Does my computer have a virus?

So i've downloaded stupid things like codec packs from possibly unreliable websites and ever since then i've been getting annoying popups and i've noticed that regular advertisements have been more perverted than usual. I use mozilla firefox and I have never had a popup until recently. if there were any... show more So i've downloaded stupid things like codec packs from possibly unreliable websites and ever since then i've been getting annoying popups and i've noticed that regular advertisements have been more perverted than usual. I use mozilla firefox and I have never had a popup until recently. if there were any they were always blocked so it never bothered me. Now they are constant. the ads that have always been on websites are now always advertising some kind of penis enhancement pill. i did i virus scan and it never said i had a virus but while it scanned the files it said something about a file with the name trojan. what can i do to make this go away, even if i can't get rid of the popups, can i ensure that my computer is safe?
Update: so i got a program to scan my computer, it did find a couple things with trojan in the filename and they were detected as high threats. I had them removed, and now things seem fine, except i get one popup on myspace. before, i would click results on google and they'd take me to different urls. so i want to... show more so i got a program to scan my computer, it did find a couple things with trojan in the filename and they were detected as high threats. I had them removed, and now things seem fine, except i get one popup on myspace. before, i would click results on google and they'd take me to different urls. so i want to thank everyone who contributed and i wish i could give you all a best answer.
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