http://developer.android.com/sdk/win-usb.html
The USB driver for Windows is a downloadable component for the Android SDK. If you are developing on Windows and would like to connect an Android-powered device to test your applications, then you will need to install the USB driver.
This document provides information about the latest version of the USB driver and a guide to installing the driver on your development computer.
USB Driver for Windows, Revision 3 (January 2010)
Adds support for the Nexus One.
The USB Driver for Windows is
available for download as an optional
SDK component. You need the driver
only if you are developing on Windows
and want to connect an Android-powered
device to your development environment
over USB.
To install the driver or upgrade your
existing driver to the latest
revision, you must first download the
driver to your development computer.
To download the driver, use the
Android SDK and AVD Manager tool that
is included with the Android SDK. If
you haven't yet installed the Android
SDK, as described in Installing the
Android SDK, please do so before
continuing with the driver
installation.
When you are ready to download the
driver, follow the instructions given
in Adding SDK Components to launch the
Android SDK and AVD Manager. From the
Available Packages panel, select "Usb
Driver Package" and download it to
your computer. The driver files are
downloaded into the <sdk>\usb_driver
directory.
After the download, follow the
instructions below to install or
upgrade the driver, based on your
needs and Windows operating system
version. If you are connecting an
Android-powered device to your
computer for the first time, follow
the below procedure to "Perform a
fresh installation." If you have
installed one of the older USB drivers
and would like to upgrade to the
latest version, follow the procedure
to "Upgrade an existing driver."
Once you've completed the USB driver
installation, please see Developing on
a Device for other important
information about using an
Android-powered device for
development.