Never open an attached file from someone you don't know.

Never open a ".pif" file.
Never open a ".bat" file.
Never open a ".scr" file.

All these contain viruses.

There are some potent viruses out there right now, and all target Windows users, and count on you to open that attachemnt - DON'T!

I'm on a Mac, so not affected by the viruses, but it's still a pain to get and delete all those files!