RE: Dell D620 and Windows 7 Pro install - (reads "Missing Operating System" - No Dell Disks (stupid idea w/o them)
Its f12 you should select. Then you should select boot from cd/dvd. Note if installing the 64 bit version that the processor is this model may be incompatible depending on the processor variant installed.
For full instructions and drivers see my Windows Reinstallation Guide (link in my signature).
Dr Philip Yip Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide