Well, since these things normally attach themselves to messages sent from your address book, they're not normally identified as junk mail. (Unless you're in the habit of sending the stuff out.)

Best not to open any attachments unless you're absolutely sure what you're getting.

My ISP has web-based mail and screening for spam and viruses is available. The virus screening is a bit over-zealous, but what the heck - I can go read the stuff in my "to be dumped" folder if I want. Probably worse than opening with my e-mail program, which doesn't do HTML.