If by stopping avast, the cpu returns to normal then surely the CPU would be logged against the avastSvc.exe process. To me that image doesn't make it clear as it doesn't attribute the CPU to a specific process, that is why I showed the Process view of task manager.
If what you say is correct across the board then I too should experience the same thing and I'm not, so there is more to this than meets the eye. If there is a conflict then the removal of one element of the conflict then no conflict. Unfortunately that doesn't identify the true cause.
Do you experience the same problems if using a different browser, firefox, opera or chrome, etc. ?
I'm using XP Pro SP3 so I'm unable to check this out as I don't have a Vista system, so I don't really know what else to suggest.
I don't know if you use a proxy to connect, but it might be worth trying this to set your browser to use the web shield proxy. This is for an older version of IE so you would have to find the corresponding settings in IE8 -
For IE - broadband users: -
Tutorial - Web Shield Proxy Set-up for IE