If you are using an SSD as your primary drive then I would be looking at changing the environment variables to that your windows\temp folder isn’t the temp location, but something like a location on your secondary drive, e.g. D:\Temp, etc. that way avast won’t be using c:\windows\temp for it’s temporary unpack area avast5 or avast location.

I believe you can also change the location of your documents and settings/ProgramData locations. You may also be able to change the location that programs are installed, you could try a google search for something like, optimising your Windows 7 for using an SSD and see what that brings (assums you are using win7). http://www.google.co.uk/search?q=optimising+your+Windows+7+for+using+an+SSD

I would have considered installing avast on your secondary drive, that however wouldn’t change the fact that it would still use c:\windows\temp_avast5_ to unpack files for scanning as it uses what you have set for the Temp folder in your environment variables. But this would mean the creation of new folders for virus definition updates average twice a day wouldn’t be on the SSD.