New iMac Freezing on Shutdown

I got a new 21.5" (standard 500GB storage, 4 GB RAM) iMac about three weeks ago, and with the exception of the first time I shut it down, it freezes on the blue blue screen with the spinning wheel every time I attempt to shut it down. I eventually give up and manually turn it off using the power button.
I quit all programs before shutdown and have checked the Activity Monitor to see if something is still running, although I have to admit I'm not quite sure what all of the information in Activity Monitor really means. I use several external hard drives and use a USB to charge my BlackBerry, but ejecting and unplugging all devices before shutdown has made no difference with the freezing up.
The only additional software I've installed since purchasing the iMac is Photoshop CS5, Blackberry desktop manager and Microsoft Office. I don't believe it's the Blackberry software that's causing a problem because it began freezing on shutdown even before I downloaded that.
What could be causing this problem? Many thanks in advance.


Welcome to Discussions - Whatever is causing the problem, you are still covered by Apple Care, so call them and have them deal with it. Their number is 800-275-2273 and they will take care of you. By the way, you have posted in the iMac G5 Forum, which is for PPCs, you have an intel machine.
Good luck,