Remove the wireless network that is triggering this error message and reinstall it - that should make it go away.
To do this, just click on the WiFi icon on the bottom right of your screen, open the network and sharing center (blued out text in the dialog box). In the Network and Sharing Center menu, click on "manage wireless networks," then click on the "remove" option. Select the network that you are having this issue with and confirm that you want to remove it.
Set up the connection again, and you should be good.