>(I have already turned off Norton Virus scanning (the Auto feature),

I had a client running norton/symantec report on speed issues with new Vista boxes. The key issue that fixed the problem was configuring the "internet security options". I dont have the exact steps handy, but under the "exclusions / signatures" feature, they had to do an add "new" and then add the signature exclusion of "spyware_netmonitor" from a very long scrollable list. They saw much better performance as a result. Hopefully adding that exclusion isnt a bad thing ;)