If you had windows 7 before you have a hidden partition on your harddrive
Or You can buy an external dvd player
get access into bios and set your cd drive as boot priority to the first and save setting insert cd and start up the system when you see "press any ke to continue" then press any button to start setup.
U have desktop remove laptop hdd and fit to desktop install using dvdplayer ,at the time of last restart shutdown system (before goto user account option )then remove hdd fit to laptop balance installation complete auto
Otherwise buy usb dvd drive
For thos that dont know a DELL MINI is a netbook with NO CD Drive....
You install Windows 7 from a USB Memory stick.... normally.