@ sina123456,
There is no need to uninstall/install the software. Follow the suggestions given to you by Logos in his post.
Very often, a simple reboot is all that is needed, especially after getting a virus definition update or after doing an program upgrade. Sometimes there is a glitch in the system and a reboot is all that is needed.
If this doesn't work, then do the Avast Repair. To to an Avast Repair:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
- Scroll down and choose Repair function in the pop-up window.
- Reboot.
Let us know if this fixes your problem. Thank you.