The wiki article Installing Games on Windows Vista+ covers the reasons and procedure for moving Steam games. Save yourself a lot of potential frustration and grief in the future by taking the time to move your Steam game (and really all games that did not come already installed) to a different location. Many unexplained problems are directly related to this location.
If you aren't aware as yet, it's pretty much universally recognized that the default install location used by Steam (to the 'C:\Program Files' folder tree) is a problem area due to the anti-malware security measures taken with it since the release of Windows Vista.
