I have been having this problem since upgrading to 5.0.462. Prior to that, I was really enjoying v5.
For the record, I am running WinXP SP3, on a P4 3.0Ghz HT CPU, w/3Gb of 2.99Ghz RAM. Lots of free disk space...
Not really sure why this is suddenly cropping up, but it's killing me. I can't do anything when Avast is running. I tried some of the suggestions like unchecking the HTTP scanning box, selecting only the IM & P2P apps I have installed, disabling email scanning, etc. As others have reported, "System" just starts consuming massive amounts of CPU.
I don't know if this helps, but I went through and disabled everything and re-enabled stuff one thing at a time. I seem to have minimized the effect quite a bit by disabling the "Behavior Shield". The web shield slows browsing down a bit, but the Behavior Shield appears to be what was killing my CPU. I have not had any other vscan packages on my system for a year or so now, at least. I upgraded to 5.0 from 4.8.
I hope this info helps, as I really love using Avast and I think v5 is a huge improvement, excepting this very critical issue.