Service Pack 1 (SP1) for Windows 7 contains previously released security, performance, and stability updates for Windows, and a few small bug fixes and minor enhancements for the OS. For example, there's a bug fix for HDMI audio devices, a fix for printing mixed-orientation XPS documents, and a fix for "Restore previous folders at logon" functionality.
SP1 is available via Windows Update, downloading from the Microsoft web site or by ording a DVD from Microsoft.
SP1 installs in roughly 30 minutes, and does require that your computer is restarted. System requirements and detailed installation instructions can also be found on Microsoft's Web site.
Don't forget to backup your computer before installing this update you never know if its going to break something.