OS Windows 7 ultimate slows down after avast update

Avast Free Antivirus / Premium Security
1

I update to 5.1.864 and everything goes solwer significantly how can I downgrade back??

2

Remove actual version and install this last one (pre-release) .884 http://forum.avast.com/index.php?topic=69152.0
See if the freezes are solved with the new version.