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?
It may be there was an error with the download, which is more likely with large files like Lion. Might be worth deleting and trying to redownload Lion before making a trip to the Apple store.