Can you get a virus by just opening an email but not downloading any attachments?

Yes.I have seen it personally (it was IE/OE 5 at a WIN2K machine that got infected with KLEZ). You must have your AntiVirus running...

If you use Outlook or Outlook Express, just downloading an infected e-mail is enough, IF you do not have all of the Micro$oft Security updates installed from Windows UpDate site, and preview turned off. With AOL, you have to at least download the attachments and open them. But it is also possible to get infected by viewing a web page if your security settings are too low or you allow the page to download things. And, most out of the box, default installs of Windows 98/ME/NT/XP/etc. can be infected by just being online to the internet and never visiting a web site or connecting to an e-mail server. So you need to have an AV, close your ports per GRC.com or install a firewall.

Also if you use outlook express you should disable the preview pane in options! just to be doubly safe!

If you are using Outlook express or MS Outlook, set security to "Restricted zone". It will prevent wild java scripts launch attached virus while reading your emails.