Seeing sabskad's thread about building a PC prompted me to double check some info I saw but was dubious about last week. Yes, it is possible to build a Window 10 computer for around $50 or less, using the Raspberry Pi II.
The Pi itself can be obtained for around $35 (
http://www.mcmelectronics.com/content/en-US/raspberry-pi). It will run off the average mobile phone charger, but for reliable performance, especially with wireless networking the charger needs to be capable of at least 1 amp output.
I have one of the first gen Raspberry Pi's. I was planning a project for my car but after some evaluation the Pi was not suitable. Not that I have the S40, I plan to use it as a "dashboard" camera mounted on the S40. Luckily the local library has recently opened a Makerspace setup, including 3D printing.
I am planning on buying a couple of the II's, one for Windows 10, which is free, and one for RISCOS, the original operating system developed to run on the ARM CPU that is in the Raspberry Pi.