Same problem and complety sure is afwServ.exe, checked the processes monitor and take 50% to 100% of the CPU.
Happens in my Toshiba Qosmio 4G-Vista Pro and in a deskop intel P35 with 4G-XP Pro Sp3.
Tried to stop firewall service but it refuses to stop.
My Avast is not free, it´s Avast Internet Security version 5.0.507. DB 100427-1, and stupid of me, paid for two years subscription...
Unfortunatly I paid for Avast. Left McAfee and now thinking of uninstall and go to another and more stable AV.
Lost money. Easy money. Isn´t it? I´m really frustrated.