unable to get exclusive lock

when i try to update i get the message "unable to get exclusive lock"
another package management application (like apt-get or aptitude) is already running please close that application first.
How do i know which one is running and then how do i close it? and what did i do to cause this error?
Thankyou for your help Ubuntu 8.04 rocks!!!!!!
oh and by the way the updates are sun java6-bin and sun java6-jre