Get rid of Norton. That's the first thing I did when I got my Dell 2 months ago. Avast and AVG work with Vista. For the Sonic drivers check out the Dell Community Forum, many posts about that issue. One thing they suggest is to get rid if DLA, It doesn't work with Vista.
I just got my new Windows Vista computer from Dell and it seems to be working good. However it came with a 30 day free trial of Norton 2007. I don't like Norton and I will not be using it after my 30 days or sooner perhaps if someone can advise me of a good alterative. And I'm also getting that same error message about the Sonic Drivers when the computer starts up.
AMD Dual Core 3800+
2 Gig RAM
Windows Vista Home Prem.
DSL Connection; DSL modem has a firewall built it.