Not really clear what is wrong as there can be more than one thing that could cause/contribute to this error.
This error in the past was often due to having another AV installed or remnants of the previous AV left behind.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
It could also be corruption within the avast installation.
Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
I don't think that is likely to resolve it, so you may need to do a clean reinstall of avast.