Have you updated your OS to the latest version? Click on the Apple (top left) and select Software Update. You may need to update to the latest version before performing other software updates. Also, are you running Snow Leopard or Leopard?
How did you get on after the software update? If you purchased your Mac a year ago it will more than likely have Snow Leopard on. I'm assuming you're trying to download and install Mac OS X Lion via the Mac App Store?