Thanks, Not you, that did the trick.
Bear in mind, however, that this let the particular attachment through
that you knew was safe, but with that choice unchecked, it will also
let through other attachments that you *don't* know are safe.
You often see advice not to open attachments from people you don't
know. I think that that's one of the most dangerous pieces of advice
you see around, because it implies that it's safe to do the
opposite--open attachments from friends and relatives. But many
viruses spread by sending themselves to everyone in the infected
party's address book, so attachments received from friends are perhaps
the *most* risky to open.
Even if the attachment legitimately comes from a friend, it can
contain a virus. I'm not suggesting that a friend is likely to send
you a virus on purpose, but if the friend is infected without
realizing it, any attachment he sends you is likely to also be
infected.
Personally I never open attachments at all, except from a *very* few
trusted sources, and then only when I'm expecting them.