Welcome to the forum.
We need more than just the problem in order to help out.
1.What is your operating System ie Windows XP, Vista etc.
2. Do you have other security programs installed on your computer? ie Norton, Mcafee etc
3. Where there any of these programs installed before on your computer?
Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair).
If this does not help, can you uninstall / boot / install / boot again?
Try the AVG8 Remover, download tool from here, http://www.grisoft.com/ww.download-tools there is a 32bit and 64 bit windows version, ensure you use the correct one.
Whilst you may have been using avast for a while, if there were any remnants of AVG left behind they could conflict, but this could present itself at any time as these conflicts don’t always happen right away.