Just to second what TC said, don't mix AV programs. I believe I was running AntiVir and wanted to try Avast. I thought I had deactivated AntiVir, when I installed Avast. Turns out that Avast does a reboot right after the install, which went fine, but then both AV's resident programs kicked in. Which locked the computer up. I ended up having to reinstall windows. For me I like Avast better, but I think both programs are good.
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