Karly,
MyDoom uses more that just zip for attachments, it masquerades as txt, bat, scr
and many others. It will install an SMTP (mail) server on your computer and then
scan your machine - text files, html files, address books etc for any email
addresses it can find and then fake some emails and spread the virus.
The attachments are always between 32-34kb in size - it's a good idea to turn
on the size column in Outlook then you can get an indication by the size of the
email before even opening it