Win XP Pro SP3 + all updates
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My computer was squeaky clean, and I had been running Avast Free A-V 4.8 without problems for months. I installed 5.0396 yesterday, and it froze in mid-installation. After several more install attempts and reboots, it finally appeared, but all shields were locked and refused to run. I used the so-called removal utility, then still had to remove about 200 registry keys manually. Some keys refuse to be removed, those with the word "LEGACY" in the key.
A second attempt to reinstall failed again. And no, there is nothing suspicious or unusual on my computer that might have conflicted with the install. This version just doesn't work for everyone. I'm moving back to AVG at least temporarily, but tell me if a fix is scheduled.