F8 at power on will access the Dell recovery partition (PC Restore). Does that not work? If not, a manual install will be required with a copy of Windows 7, 64 bit. You can download an iso and burn it to a DVD on a PC and use that and the Dell Windows 7 Product Code. You will need some Dell supplied drivers
(at least Sound drivers) and you can get them at www.support.dell.com
using the PC's Service Tag number.
Win 7 iso download links, get the exact same version as what you had. e.g. if you had Windows 7 Home Premium 64 bit then get that iso. Official Windows 7 SP1 ISO Image Downloads