Another use is to install some variant of Linux and run it as a NAS (or media server, which is a specialized NAS). Before I switched to using COTS NAS hardware - Synology, specifically - I had a lower-spec desktop functioning as both a NAS and as the primary gateway for our internet. The latter is useful as it gives you more detailed control & better security, usually, than the consumer grade routers. (Currently, our gateway device - which is a consumer grade router - is running DD-WRT, an open-source router distribution based on Linux).
On home networks, NAS devices are useful both as media servers and as shared storage for the people at home. EDIT: Oh, and as a first-level backup device for your computers & portable devices; check Reslio Sync, among others, for that usage.
In so far as trying to run Windows on it, I can't say; I don't know enough about the requirements or your CPU.
I think there's also a flight simulator that runs on Linux, but it is quite different than Microsoft's Flight Sim, and probably much more for hardcore folks with a full HOTAS setup and the willingness to tinker with both S/W and hardware.
