Avast DOES scan e-mails (and attached files) before it gets to your e-mail client. It works as a proxy. Avast proxy works for POP, SMTP and IMAP protocols. The only thing is that i don’t think it scans for newsgroups. For script blocking, you have to use avast pro version. The home version does not provide script blocking.

The on-access scanner, by default, only scans for file being executed. So it should not slow down your computer. It does not scan files on creation, modification, read, write… If your computer is still slow you can disable the on-access scanner but i really don’t recommand it !!