Mithrandir,

If you just want to get rid of everything on you HD, R'as al Ghul has already explained the way. However if you want a new, fresh Windows, then setup of Windows will lead you to formatting of your drive when you choose to install Windows -not the recovery console- in setup's first option screen. And when it propmts for which file system to use, choose "format using FAT file system". Since your PC is old a bit that you may face compatibility issues if your system fails in the future. However NTFS is a more optimal file system in case you want to try it. I'm using it peacefully.

Good luck