I don't care about company!!
Me too... I've used Symantec, I talk about my personal experience.
Anyway, using in a Virtual Machine, *any* antivirus will run slowly and the machine itself will be slow.
Will you test it into Windows 7?
I know it would be slow and it's a good point, because you can feel how much resource would it use, you can feel it better.
I would test it in Sun VirtualBox and it's not slow like Microsoft VirtualPC or VMWare...
usually first I test in Windows XP and then Vista, but, this time maybe I test it in a Windows 7 too, and not Virtual, make my System Dual Boot with windows 7 and try avast! and Norton (and maybe avira and...) in that windows
after done it, I would share result in my computer security software blog:
http://getrideofit.spaces.live.com/it would be my first post that I've personally tested