There is so much I don't know and I just keep right on stumblin' on to such things as this. Where do I get the Win-XP . OS

Today will be 10 days since I've been "cramming" on how to build a PC - - - I'm sure there's so much more to learn I'll be doing it "on the job" BUT only with your help.
As always - - appreciate your help and patience, caveman