Taking you at your word, the easiest way to delete everything except windows, is to export all of your settings (assuming they still have that), reinstall windows and freshly format the disk whilst you do it. This will leave you with just windows which is exactly what you say that you want. BUT there is zero chance of getting anything back if that is not what was really meant, no files, no folders, no music, no photos, no complex bank passwords. etc. etc.