Another way to do this is set up your POP account to forward all you emails to your gmail account. I do this because I usually leave outlook express (I know, I should use something better ) open on my PC and it pulls down all my e-mail, so if gmail is checking it, it won't find anything. Doing the forwarding ensures that all the emails get to gmail.
Glad to know I'm not the only one stuck with Outlook Express. My boss still uses it as our mail client at work! Actually in the process of switching to Outlook as I type. It's about time.