I updated my ais to 5.1.864 and what happened was something similiar with the problem of this user : http://forum.avast.com/index.php?topic=69071.0
Hello and Welcome: You might want to try version 5.1.874. This version is pretty stable prerelease and has fixed most of the issues experienced in .864
If you notice the download link says 5.0.874 that is not correct as when installed it will read 5.1.874
This downloads is not available via GUI so I suggest uninstalling your current version and Install 5.1.874 from the installer link here: Internet Security: http://files.avast.com/files/beta/5.0.874/setup_ais.exe
Let us know how it goes. 
schmidthouse,
already recommended that on the Portuguese board but still without success and even with a BSOD.
http://forum.avast.com/index.php?topic=69103.0
OH, OK Thanks Silk0. :
To bad I don’t know Portuguese :-\
@ Silk0,
You may want to report your problem in the http://forum.avast.com/index.php?topic=68927.165 thread. Thank you.
Not my problem.
Everything OK with me.
Now… i need to send a PM to the OP with instructions to report the problem.
There is a new pre-release refresh build 5.1.884 if ana moreira wants to check that out.
By all accounts it shouldn’t be long before this becomes a regular program update. http://forum.avast.com/index.php?topic=69152.0