Common formats are .jpg or .gif for images, .zip or .sit for compressed files, .swf or .gif for animations, and .exe for those cute little programs that are time wasters, usually. These basics are just a first step but they'll go a long way in protecting your computer. You and Anna viruses were written and distributed in such a way that they take advantage of people's emotions and curiosity. They spread themselves by sending themselves out of their victims' email accounts to everyone in the victim's contact list, thereby successfully circumventing the first rule above. If their victim's had practiced the second rule, they would have been fine as each of these viruses' malicious attachments had a .pif extension.
Some unlucky folks see more spam than legitimate mail in their inboxes.
Most email clients, like Eudora, Outlook, and Outlook Express have mail filtering ability. Refer to the Help files in these programs to see how to activate and utilize this feature. See, chances are that your free service isn't as free as you thought it was. The price usually ends up being your demographic information and email address, which can be sold to other companies and so on and so forth. The 'nut' of it, as a Georgian I know says, is that your inbox can be a very dangerous place.