I doubt this has anything to do with any kind of virus/malware infection. You aren't the only person that got hit with this the moment the new Avast update rolled out. I had the exact same situation on Win7x64 Ultimate - IE, Firefox, some desktop gadgets and games immediately stopped working. Fortunately Chrome worked about 1/2 the time and I could use it to get the avast cleaner app.
In another thread somebody indicated that they succeeded after cleaning out avast, and then manually installing the current version instead of allowing the update to happen. That didn't work for me, even with disabling Threatfire and SUPERAntiSpyware first. Fortunately I kept my 5.0.677 installer, and that still works.