Chavarin: Welcome to the avast! forum. I am not aware of avast causing a sound card problem. Try uninstalling and reinstalling avast to see if that solves problem. Next time you post please add info on what other security programs you are running so we can help narrow down the problem. Please tell us exactly what the error message is. If you have a problem translating the error screen to english, tell us what it says in your language, someone here should be able to translate. Glad to have you aboard.
I have installed avast on a Win98SE system with no problem. I did have a Windows update kill the sound however.