I do not use avast, So I can not say if this normal. But I did check mine .It use 0% . Unless it is scanning then maybe 15%. You could try removing Avast. With its Uninstaller program. The reinstall it. See if this helps. It may have a corrupt file. Or you could try using another antivirus program.
Do as Cobalt006 suggested then install Avast from here http://www.techspot.com/downloads/223-avast-home-edition.html
I use ESET Nod 32. I have used for years now. I don't mined myself paying the dats update . It has served me well.
I use Avast free version from the link I posted.
I have no such issue, but then I also don't run scan and only enable email and weblink validations.