As mentioned by Para-Noid try a repair. If it doesn’t work uninstall it via Add/Remove, run avast uninstall utility, and download and run Avast Cleanup Tool found from here http://rejzor.wordpress.com/avast-cleanup-tool/
Note: Run Avast Cleanup Tool in normal mode. After using Avast Cleanup Tool, restart your computer manually. After that download and install a fresh new copy of Avast, register your copy of avast, and restart your computer manually again.