Installing the android usb driver is the #1 priority if you ever want to access your phone through a computer, be it to transfer files or executing commands. This could lead to a lot of confusion and headaches if you fail to do this process.
Note: Newer versions of windows(vista and win7) will automatically install them as basic mass storage devices. You do not want this!!!
Firstly download the file android_usb_windows and unzip to your computer. Go enable debugging on your phone. (Settings->Applications->Development->USB debugging).
For WinXP
A window should pop up whenever you plug your phone into your computer, asking for you to install the device. Select “browse my computer to install driver software” option. Follow the steps to update the driver and include the android_usb_windows in your search when given the check box.