I got it tonight too, I read all the previous posts and it looks like nothing has been done about this yet, except the guy that found out you could downgrade to IE 8.0 from 9.0.
So it sounds like the issue isn't related to paypal and avast entirely. It sounds like an issue related to Microsoft's IE Explorer 9.0, Avast, and Paypal.
Since it's Internet Explorer related you could downgrade or just use a different browser. And for the record, I am not getting these messages using Google Chrome, which is an internet browser like Internet Explorer if you didn't know what Google Chrome is.