If you had another AV installed when you installed avast it could cause conflicts, causing a corrupt installation. But the normal add remove programs if an entry was created should remove all of of avast. Otherwise you would need to use the uninstall tool as Bluesman mentioned.
1. Uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot.
If step 1 above failed it may be necessary to run this from safe mode (or the self-defence module may block this), once complete reboot into normal mode.