you need to format the hard drive and install windows, when you got the pc you will of got a windows cd or recovery cd or there will be a image on the hard drive if its a cd you need to go into bios and set first boot device to cd
to go into bios you press del on the keyboard when you turn the pc on if del doesnt work try f11 then when in bios set first boot device to cd then save and exit put windows cd in your cd drive and boot cd then its just a matter of following the instructions ie format in ntfs and install windows