Multiple Infections

My computer has been infected with Windows Police Pro, Antivirus Pro_2010, Personal Guard, Green AV, Windows Security Alerts, and Windows Security Center. I tried to delete Windows Police Pro and Antivirus Pro_2010 on my own but they kept coming back until this last time, but I'm not sure they're completely gone. The problem started on Sept. 4 when two fake alerts popped up and I had to restart my computer. After I restarted I noticed that in Opera if I did a google search I would either get a blank page or I would click a link and be taken to a new tab for another search engine. I did a scan with McAfee which found 11 items on my computer. The items FakeAlert-Gv, Generic Downloader.x!bff, Generic Downloader.x!yj, Generic.dx!bbc(found twice), and Generic Downloader.x!bcj (found 4 times) were quarantined; PrcViewer was detected but I removed the file; And Generic Rootkit.d!rootkit which it said was removed. I did a second scan which found nothing except the rootkit which it said it had removed. Google still wasn't working in Opera, but I decided to upgrade to latest version anyway thinking that would help. I also decided to download updates to rest of my computer. After I did that I to get popups from Windows Police Pro. I tried to download Malwarebytes' Anti-Malware but I couldn't open the set-up until I changed the name. When I did that I got it to install, but I still can't open it. I also cannot run HijackThis. I am using Windows Xp. Please help.

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