A human brain can be used as a general purpose computing device. Invent and produce low tech stuff like paper, pencils and an eraser and use your humans to execute instructions and check each other for errors. Alternatively use sticks and sand. The computer would run very slowly but it would run. 8 bit assembler for windows 1 code is not that difficult to get your head around, its just tedious. I recon you could do this inside a year.