Go to the Gmail webpage with your PC and log-in.
In the upper right-hand corner click on Settings.
On the Settings page, click on Forwarding and POP.
Next to POP Download 1. Status: Does it say POP is enabled?

If not, Check Enable POP only for mail that arrives from now on.
Select an option for #2 (I keep Gmail's copy in the Inbox)
Then click Save Changes.

There's a link in #3 that shows how to configure most email clients (I use Thunderbird 1.5)