This is the Windows XP and Windows 7 64bit driver for the Samsung Galaxy S USB driver. Note that this was the last driver released for the phone so it is the most up-to-date. There is also a more generic USB Driver for Samsung listed here.
First your will need to download the file and click on the .exe file. This will then install the driver. Always disconnect the phone before you install the driver. If it does not work then restart the computer. If that does not work then try different USB ports on your computer. Should that not work then see here as to what the USB problems might be. Most installs are easy and work the first time. There are always the odd ones out.