Oh c'mon, something like this would be a great feature for people that often forget what they've installed on their PC. Now you can just call Microsoft and ask them. 
There are a few serious problems here. The big problem is that Windows 8 is configured to immediately tell Microsoft about every app you download and install. This is a very serious privacy problem, specifically because Microsoft is the central point of authority and data collection/retention here and therefore becomes vulnerable to being served judicial subpoenas or National Security Letters intended to monitor targeted users.
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