You say that you already have another anti-virus that is working. Installed on the same machine as the failed Avast uninstall?
Left over pieces can conflict with other Antivirus programs. You must uninstall properly. Revo free has always been excellent for properly uninstalling in the same manner as 'Programs and Features' does, then scans for left over pieces. Just look closely at it's leftover pieces to insure they are the Antivirus in question.
Personally I would uninstall all AV's and start from scratch. Most AV's have a seperate downloadable uninstaller to remove leftovers after a proper uninstall procedure has been done. If my first sentence is true, your machine could be locking up to some degree. Avast will tell you to use it's separate uninstaller in Safe Mode for proper usage, this could be true for whatever other AV you have.
Definitely make some restore points before you do anything to go back to. When I get a machine running just how i want it, I image it completely before any AV so I have a clean baseline to always go back to. Macrium free has never let me down.
Hope this helps