A lot of people have been having trouble installing the Nexus S Windows USB driver, so here’s the simplest way, just download this file:
Download samsung_driver.zip
Unzip the file then run the .exe file.
After you install the Samsung drivers, put your Nexus S into Debugging Mode by going to Settings->Applications->Development then checking Debugging Mode.
Connect your phone to your computer and open up Device Manager.
You will see Nexus S with an exclamation mark, just double-click on it, Update Driver->Browse my computer for driver software->ADB Interface then choose Samsung Electronics Co., Ltd, then choose Samsung Android ADB Device.
UPDATE: If you can’t find ADB Interface, try selecting “All devices” instead of ADB interface, this will list all drivers and you should be able to find ADB.
And you should be good to go.