On a Dell Vostro 1520 series. Holding down the F5 key
when powering on should work.
It takes you to a menu where you choose the system restore option.
Or else - These are all from the link provided by CatByte (your Helper) http://www.ehow.com/how_4778078_reformat-dell-laptop.html
Tips & WarningsFull formatting destroys all data on the computer
Back up any data on the PC that you plan on saving. This can be accomplished by using burnable CDs or DVDs, attaching an external hard drive, using an online backup process or hooking up to the corporate backup server.
Good luck -