If it's a web based email service like gmail, yahoo etc then change the password immediately.. use a strong one with 12 characters minimum, upper and lower case, numbers and a non-alphanumeric character or two. If the address book is stored locally like Outlook or Thunderbird for eg then it would be a good idea to do some scans on the computer to make sure you haven't picked up some sort of malware/exploit.

Check the thread at the top of this forum for the recommended scans..

http://discussions.virtualdr.com/sho...d.php?t=167915

Also, have a look in your email outbox and see if any of the sent emails are there.

Password strength analyzer.. http://www.passwordmeter.com/