Reformating Hardrive / Reinstalling Windows?

Ok, my mums pc is acting up and also it a bit slowish now. I want to have it so it like new so to speak. Im a n00b when it comes to this area. I want it so only the default programs are on there and no data i created is there. What would I have to do Reformating Hardrive or Reinstalling Windows or even both?

If so can sombody please give me step by step instructions on how to do this thanks. I mean like what kinda drive like NTFS or whatever the size, I dont know any of this so i require an idiots guide cause I know nothing.