No need to be worried 8)
If you have Windows XP\2k, avast 4.5 has a new mail detection that will work automatically. Please, set up your email account just as avast does not exists. It will detect the pop3/smpt/imap ports and scan your messages.
If you have Windows 98\Me you should change to 127.0.0.1 or run the Mail Protection Wizard.
Can you post more information about your system? 