There is a conflict with the Mac App Store and Macs with Boot Camp installed, where downloading applications or updates in the App Store may fail or become unresponsive, or applications showing as installed in the App Store refuse to launch. Apple is aware of this problem, and is working on a fix, but their official workaround posted to Knowledgebase article TS3640 is to add the BOOTCAMP volume to the Privacy tab in the Spotlight system preference pane. Another confirmed workaround is to use the Disk Utility to temporarily unmount the BOOTCAMP volume, or right click on the BOOTCAMP volume and select Eject.