What previous antivirus did your computer have? Norton, McAfee, etc. Run it’s corresponding uninstaller/removal tool in safe mode. Did you re-install your previous antivirus again? Since you uninstalled avast, I suggest you to do this. After you run the previous antivirus uninstaller/removal tool. Run avast uninstall utility in safe mode, after that download and run this tool in normal mode Avast Cleanup Tool http://rejzor.wordpress.com/avast-cleanup-tool/, after that manually restart your computer again.
Finally install a fresh new copy of your Avast Antivirus. I hope this will work for you!