It sounds like you had some pre-installed software bundled with your computer which you used for the webcam. This is put on by the manufacturer (Acer, Samsung etc). When you do a re-install with a standard Windows 7 Disk none of this software gets put back on, just Windows.
Can you remember what the software was called? Normally the software is Cyberlink YouCam but there a loads of others that it could have been. Quite often they are not free programs either
Also, what model of computer do you have? Might be able to figure it out from there.