It may be possible to contact Sony and receive factory restore disks directly from them. I do not know this for sure, but it may be worth exploring.
In terms of doing it on your own without contacting Sony, the only way to restore a PC to factory is to have the software/disks to do so. The issue with recovery disks is that they are HDD and BIOS. They are not linked to the case, or even the CPU most times. So the only way to obtain these disks is to have them either sent from Sony, as stated above, or have them made by someone with the SAME computer that you have, that has the SAME bios. Otherwise the disks will not initialize installation, normally displaying an error message regarding you having the wrong computer.
The best case scenario for you is to install Windows 7 from a basic install disk, then use the key on the bottom of the PC as stated by faalin. There may be an issue with using that key, however. The key is OEM, meaning that it is meant for a pre-install or recovery ONLY. It is common for it to come back as not genuine after performing an aftermarket install of windows, even if you have done absolutely no hardware changes.