AVG 8.0 shouldn't give you problems. It might just be that it's getting hung up a bit on initialization or when trying to do a virus scan on start up ... not really sure.
Sometimes users have too many applications loaded on startup, or not enough available resources for everything they run. Disabling some apps could make a difference. If you want to stop one or more programs from running on startup, try this:
click the Vista Start icon -> click Run... -> type msconfig into the dialog box; click OK -> click Continue if\when the User Account Control permission box appears -> click on the tab labeled Startup -> uncheck the boxes to the left of the programs you know you do not want to run on startup. **Sometimes disabling startup prgrams can affect accustomed software functionality. Try disabling only those programs whose functions you understand and will not miss.
Otherwise, I've had good experiences with
avast! in Vista. You could try switching it up. But like T I M, I've never had problems with AVG before (I've used numerous such programs over the years).
EDIT: Also, one thing I've noticed with Vista is that it takes somewhat longer to boot up and fully initialize than XP. I read somewhere that this is because Vista takes a bit of time to put more of the regularly-used processes into quick-access memory. This is designed to make the overall computing experience much smoother; though, again, it does take more work on startup.
One thing you could try, knowing this, is to wait a little bit longer for Vista to load before you start trying to surf the web, etc. I know it's annoying, but giving it an extra 30-60 seconds could allow it to settle down the processor usage, freeing up resources for whatever you're trying to do.
Hope you find a fix!