Slow and unstable startup and operation

I don't want to insult anyone's computer savvy, but I wonder if any of the reports of sluggish and unusable operation of 5.0 might be caused by the installation of Parallels Internet Security (acronym) in conjunction with another antivirus application. I tried this with MS Security Essentials already installed in XP, and the results are disastrous (even though at first it seemed unchanged). Almost any operation, after a very long startup, got sluggish beyond belief, and most would eventually hang. Removal of (acronym) put everything back to normal. I would guess one could alternatively remove the other antivirus and keep (acronym) with the same result, but haven't tried it. It is pretty well known that most antivirus applications don't play nice with each other.