Do you have the copy of the product activation code that came with it (ie. it is usually attached to the case) and is the key readable ?
IF you do then all you need to do is download the windows 7 iso that fits the version of the OS that you have (ie. Home Basic, Professional, Ultimate) the key for (Links to available for downloads HERE - http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/ ) (NOTE : Digital River is Microsofts approved source for digital downloads so these are legitimate isos)
Once you download the version that matches your key - burn it to a disk and use it to boot the system and reinstall typing in the key from the certificate when requested.