It’s not open source, but Tumblr is a decent option if you want something fairly easy to setup. Additionally, it can look very professional with a good theme..
As others have already mentioned, Pixelfed is probably your best option for a FOSS solution to your problem.
The best I can offer for your original question is to delete everything you don’t want and then run something like BleachBit to shred the data. As for the second question, if a computer originally came with Windows then it should automatically activate after reinstalling.