Windows 7 complains that Nexus S driver is missing
This is a common problem among Android phones. Sometimes Windows might try to install drivers for your phone which might not necessarily be complete.

Like Matthew said in the answer above me, you could use the Android SDK package which usually includes drivers for Nexus devices but in the event it doesn't work:

You need to get the usb drivers from the manufacturer. Following the instructions in this link might help:

http://nexusshacks.com/nexus-s-hacks/how-to-install-nexus-s-windows-usb-drivers/