Yeah, it's not just you man. I've got two machines that I just went through fresh Windows installs on
Shutdowns were extremely quick, like 5 to 10 seconds, before installing Avast on it.
As soon as i did install it on both of them, about 50%-75% of shutdowns would take around a minute. Greatly noticeable.
And I also tracked it down by enabling Verbose Status for Logins, Logoffs, Shutdowns, and Startups via registry hack that can be found online
When i did that, it would always hang at shutting down the Avast service.
And for clarification, the machines I worked on are Windows 7 Home Premium 64 bit.
Not sure what code changes were done recently, but hope they're fixed