First of all, if the mail you opened has got a virus, it's probably a Windows virus, so it won't affect to OS X. However, you should run a scan. To do it, you can use Sophos.
As it's probably a Windows virus, this won't do anything, but open > Software Update, and install all available updates for your computer. If you want more information about OS X and malware, see this Thomas A Reed's site > http://www.thesafemac.com/mmg
For scanning incoming emails ClamXav does a great job.
You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.
More useful information can also be found here:
Stupid me. Opened email attachment, need scan for trojan or virus.
Unless you are a known Tibetan sympathizer or use Windows on your Mac, there is nothing to worry about. What kind of attachment was it? What did it open in?
I had Tech Tool Deluxe, Sophos.
TechTool Deluxe would not help you with any type of malware issue. Sophos should be able to tell you what the attachment is if malware.
Be cautious with regard to potentially infected e-mail. Never use A-V software to move (quarantine) or delete e-mail. It will corrupt the mailbox index which could cause loss of other e-mail and other issues with functions such as searching. It may also leave the original e-mail on your ISP's e-mail server and will be re-downloaded to your hard drive the next time you check for new mail. Use your e-mail reader's delete function to prevent these issues.
If you've got Sophos installed, and haven't disabled its On Access scanning, then it wouldn't have allowed you to open a malicious attachment. You would have been prevented from doing so. Unless, of course, you're the lucky discoverer of something brand new, but that's fairly unlikely. The Mac malware world has been fairly quiet lately, except for a few things that are specifically targeting the Uyghur people of Asia.