If you disable avast then your AV protection wil stop....
There seems to be a problem at the moment with the update to version 7.0.1.456 not completing properly, with it making the machine hang at startup
I've found the best way round it appears to be to go to Control panel > programs & features (or add/remove programs depending on windows version) and then use the add/remove feature to REPAIR the program
Reboot, give it a few minutes to update and sort itself out and it starts behaving again
Its important that you don't uninstall Avast in an attempt to fix it, as that screws up registry and file/folder permissions well and truly, stopping reinstallation. I've yet to find a way round that, and from Avasts replies to my questions, nor have they