What antivirus software did you have previously? Did you use it’s uninstall utility/removal tool?
As for your avast software? Did you click repair via add/remove? If you did and still didn’t work, please do this uninstall Avast via add/remove, run this http://www.avast.com/en-nz/uninstall-utility.
You also could or at your discretion run this tool http://rejzor.wordpress.com/avast-cleanup-tool/ . Run this tool in normal mode (not in safe mode)