I have the exact Same Problem!
I installed avast internet security 6. After installation and a computer restart(as prompted to do so), the computer just doesn't respond. It will start up and everything looks fine, but once avast starts (as it's a start up programme) it all just freezes. It's was rendered useless. So I restarted in safe mode and restored my system to the state b4 i installed avast.
I have now done this several times, initially i thought it was not avast, but I'm sure it is. My computer is brand new, and has windows 7 64 bit.
I paid in full for a 2 year internet security subscription.... WTF.... so not cool...
Anyone know how to solve this?