I have yet to come across an anti-virus program that can update itself without bringing at least some computers to a halt. I've tried Microsoft Security Essentials, AVG and Avast (and read many posts here, and elsewhere, about similar problems with full commercial software) and used XP, Vista and 7 (both 32-bit and 64-bit) and still never found anything that doesn't result in Firefox (and some other programs) producing 'not responding' messages while the anti-virus program updates itself.
So it's quite likely to be Avast itself that's
causing the very problems that you're referring to! (I'm not knocking Avast though. I use it myself because, for me, the problems occur far less often than with other security software but I've still had a 5 minute wait, with everything at a halt, only a few minutes ago while it updated itself).
My post here explains how you can (hopefully) seek to track down which background program is causing your problem:
http://www.theanswerbank.co.uk/Technology/Computers/Question1425027.html
(That thread was about Firefox but my post still relates to your problem as well).
If, as I suspect, the problem is related to Avast updating itself you could try uninstalling and reinstalling or possibly trying AVG instead. (I ditched AVG precisely because it was causing such problems frequently for me but you might get a better result).
With regard to Avast Grimefighter though, my advice is simply 'Don't bother'. It's a registry cleaner, which the technical press generally say are best avoided. However, if you want to try one anyway, the one registry cleaner which is generally well-rated (both in the technical press and by AB members) is the free version of CCleaner:
https://www.piriform.com/ccleaner/download