Hello annelithern,
You have posted in the wrong section of the forum; you should have posted in the Avast Free/Pro section. But we will help you here.
It would help when you post if you could give us the Avast product you are using (Free, Pro, AIS) as well as the version you are using and your OS so that we can help you better.
For now, I would do 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. Also let us know if your machine is acting normally after this.
If the Avast Repair does NOT fix your problem and you have just upgraded to a different version, then I recommend an uninstall and clean install using the Avast uninstall utility tool
http://www.avast.com/uninstall-utility, but again, we need more information from you prior to giving you more instruction. Thank you.