WiFi issues MacBook Pro

I am having to rejoin my home network every time I start up my MacBook Pro or my iMac. I just had to replace my router (Belkin "Pre-N") after 10yrs of hard service. I got my self an Airport Extreme. The other macs (iMac and iPad both the wife's) are having no issues was so ever, all are using wireless connections. I just did the software update to the airport utility and lion. The issue pre-dates the updates. Anyone got any Ideas on a fix. I set up all machines the same.
Thanks for the help.

Did you remove the old router from your network preferences?
Go to System Preferences>Network>Advanced (button on bottom right)>Wi-Fi tab. Select the old router in that list and click the "-" sign to remove it.

You may also want to configure the Location settings while you're in the Network settings. I have mine set to Automatic and it always connects to my home network no problem.