Hi, I had the same problem.
I updated from Avast 4.8, uninstalled it and installed version 5, all the latest updates. Everything became very slow - especially browsing through IE and file browsing through Windows Explorer.
When I disable all the shields it runs great - Vista Bus, 3GB of RAM. I'm a tech and have installed hundreds of instances of Avast.
I turned off all the shields and turned them back on, one by one. The problem shield is definitely the 'behaviour shield'. I turn this off and it works fine.
If anyone knows any reason why, please let me know.
Hoges