Installing 64 bit drivers on Windows 7 for Nexus S
Download Android SDK.
Using SDK manager, download Google USB Drivers. Take help from www.android.com for "How to do that?".
Right Click Computer -> Manage -> Device Manager -> Right click your Nexus -> Update Driver -> "Browse my Computer for Driver Software" -> Browse path "C:\Program Files (x86)\Android\android-sdk\extras\google\usb_driver" also tick "Include sub folders"
You're good to go.