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#11
Almost certainly No.1. Memory leaks are without question deficiencies in the OS, so it's a Microsoft software problem
It would be very rare for system software to be responsible for a memory leak in a released product. Almost always it will be due to an application, driver, or malware. Even when the leak is in a process belonging to the OS the cause is usually external.
A memory leak will not necessarily fill all of RAM. The leak is actually in the process private virtual address space, not RAM. This may produce high RAM usage but that is just a side effect. In fact, the process with the memory leak may not even have high RAM usage. Explaining that is not simple and I won't even try.