I am taking it you are installing windows 7 from a DVD you put in the drive to install windows 7.
If that answer was Yes. Then we will move on to the cause as to why it does not show in windows once it has loaded. And while we are about it if you can tell us how many hard disk drives are in the system, and if they have been split and partitioned into smaller drive sizes.
Have a look in windows and list all of the drive letters you can see through my computer.
C: D: E: F and so on. You may have what we call a drive letter assignment clash.
Where for example a hard disk drive has the letter F: assigned to it.
But the DVD drive also wants to use that. The end result is one simply does not show in my computer if the case. As a drive letter cannot be shared, for a device be it a HD or a DVD drive. So it will knock one out looking like it is missing. It cannot run two devices requesting or set to the same drive letter Assignment at the same time.