Well, I’m afraid I don’t know what to suggest… if the problem occurs even in Safe mode, I think it isn’t really caused by avast!, because avast! drivers are not loaded (in safe mode).
According to what google returns for the mentioned error, I’d say that there’s a problem with reading some critical file from disk (i.e. a hard disk problem). Isn’t there any path in the error? - something like
c0000221 Unknown Hard Error 1\SystemRoot\System32\ntdll.dll
?