Start | Run | Type regedit, and click OK | Expand the following branch...
HKEY_CURRENT_USER | Control Panel | Accessibility | StickyKeys
Check to make sure the Flags String name is set to a Value data of 506. If the Value data needs to be modified, right click the Flags String name --> Modify | In the Edit String dialog that opens, make sure 506 is in the Value data field.