If you don't know who the email is from...
In general delete it... but remember the email could have come from someone genuine such as a friend with a new address or a prospective clients, think before you delete...

Beware of attachments!
Some attachments contain viruses, like the above advise, if yoou don't know the sender delete it!
If you know the sender scan it with an anti-virus program.

Create an "web only" email account.
If you have create a "spare" web only email account you can can give that one out instead of the one you use for daily use. Use this when a page demands you sign up and give an email address.
Better still use a service like YAHOO! who not only offer Spam filtering but antivirus protection as well for free.