Unable to access account on 2nd computer

My MacBook recently stopped allowing me to access my Apple account within iTunes. I get an "unknown error (-9807)" message that suggests making sure my network connection is active. I can see the iTunes store items, I can access the internet, and I can send email so connectivity isn't the issue. Anyone else having these problems?
I can still access iTunes via my older PowerBook G4 just fine and my account still shows two computers being authorized. Suggestions?