We've all seen mysterious .dat file attachments in our inboxes from time to time. In most cases the .dat attachment originates in Microsoft Outlook, which produces outgoing messages using a slightly modified version of the Rich Text Format. This action helps to preserve fonts, formatting, and the like, but it often causes problems for the recipient. If your e-mail program does not support this particular blend of RTF, you'll end up with that danged .dat attachment--and no easy way to open it.
You have a couple of options. First, you can ask the sender to turn off Outlook's RTF setting in favor of plain-text e-mail and then resend the message. Second, you can try opening the attachment in Notepad. You may see a lot of garbled-looking code in there; but if you comb through it, you should be able to find the core text. Finally, try WMDecode, a free utility that will scan the Winmail.dat file for attachments (and only attachments, not the formatted version of the e-mail) and save the items along with their original filenames.
(For more solutions to perplexing PC problems, see "The 21 Greatest PC Mysteries--Solved!")