I think you may be referring to the PXe Boot. You need to press the F12 To boot to LAN but the PXE server must be configured properly so you can get the image from there.
Make sure the Boot to LAN option is enabled in the BIOS.
I am referring to a list of drivers required to access the network from the booted Microsoft Windows 7 Ultimate 64bit repair disk.
As apparently needed to read an image from a network location.