have an OS that is at the same price as the current models but make it so that the ONLY applications that can be downloaded are through the Windows Marketplace, if Microsoft were to expand the marketplaces massively?

Surely this would decrease viruses, still make them money through the submission of the apps, and allow them to have full control of the PC? ****i do not mean ONLY Microsoft apps but all apps!***

Does Apple Macs already have this because i heard that there are limited 3rd party apps for the macs for some reason?

Cheers for any answers, however don't argue whether Windows is better than Macs because they clearly are :P