We will need more information to be able to help you:
Which OS are you using? Is it up to date?
Do you use a firewall? Which one?
Do you have any other antivirus installed in your system?
Any other security programs that could interfere?
Which is your Windows? 98, Me, XP?
Do you use any other antivirus in 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). You must be connected to the internet while repairing.
If this does not help, I suggest an installation from the scratch:
Did you ty the uninstall utility? If not, follow the first 4 steps in tech’s post. Since you have nod32 installed, do not reinstall avast. 2 avs usually don’t play together nice.
If you have another antivirus in this computer, remove it first.
If even though the Control Panel can’t remove avast, follow the step 3 on reply #1 above.