No, that’s not a virus. And no, there’s nothing with a .dat attachment on my emails, even though Outlook Express wants you to believe it. It’s a digital signature. It looks like this:
Content-Type: application/pgp-signature
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.2-nr2 (Windows XP)
Comment: Using GnuPG with Thunderbird -http://enigmail.mozdev.org
iD8DBQE/fSqo2FTH6fnzT0IRAv81AJ42L5GD3XrqN0Sv/
ksXaKvcOCuQhgCeOlgy9FaROdCRoGB3Qd6lunjKDvk=
=NHUe
-----END PGP SIGNATURE-----
What it does is to confirm that this email is actually written by me, and noone tempered with it. It’s like putting my signature under a written letter, only more safe against tampering.
Why is this such a problem for some people? Outlook Express. This degenerate piece of shit software decides that “if I don’t know what it is, I’ll make it look funny, invent an extension for it, and oh, while I’m at it, I’ll also not show my user the plain-text parts that I *could* understand, but pretend it’s in a .txt file attachment. And on the default security settings, it removes them:

If you’re an Outlook Express user, this is pretty annoying. But trust me, the mails from Outlook Express users are painful, too – and not because of a fault in anyone’s mailtool but the sender’s. So if it’s okay for the OE users to send their non-standard borked emails, I don’t see why I should stop sending my perfectly standdard-compliant emails just because OE doesn’t want to handle them.
“But Enno”, you’ll say, “don’t you care about whether people can read what you write?”. Yes I do. But the people I really care about are tech-savvy, and they don’t use mailtools that are old and broken. The standard for MIME (RFC 2048) will be 7 years old next month, and standard for OpenPGP/MIME (RFC 2440) will be 6 years old by then. They were practically invented before Microsoft realized that the Internet was something that mattered to them. Plenty of time to implement them. And about time to shrug, shake your head and go on with life. OE is dead.