The Hibernate function on computers can be quite useful. When resuming from hibernate you can start right away where you left off with all programs open as they were when you stopped working and frequently the resuming from hibernate is faster than starting the computer after it was shut off.
You will still have to periodically reboot since Windows does get slower after some cycles, but that can be many days.
Many laptops have this function enabled when you are in the shut down menu, but most desktops do not.
For Windows 7, have a look at http://www.techlikes.com/2010/04/25/how-to-enabledisable-hibernation-in.html# or http://www.intowindows.com/how-to-enable-hibernate-option-in-windows-7/.
For Vista look at http://www.howtogeek.com/howto/windows-vista/re-enable-hibernate-option-in-windows-vista/ or http://support.microsoft.com/kb/929658 and for XP at http://www.giannistsakiris.com/index.php/2009/04/08/how-to-enable-hibernate-in-windows-xp/.