If you're a desktop gamer who never uses Windows 7's hibernate function this tweak is for you. When you hibernate a laptop, the state of your open programs and hard disks is saved on your hard drive so that the computer can be completely shut off. Sleep - used more often by desktop users - puts the program states in memory and draws a small amount of power. You basically always want to use sleep if you're not concerned about saving battery life since it's faster to read from memory than a hard disk. Windows keeps a large hidden file on your drive to store this data when hibernate is used - hiberfile.sys. On a PC with 8GB of memory I've seen this file take up 6GB. On a 4GB PC it seems to be 3GB.
Open up a command terminal by holding Windows Key + R and typing "cmd" in the Run prompt. Type "powercfg -h off" and the hibernate feature will be disabled, deleting the hiberfil.sys automatically. This can be reversed by typing "powercfg -h on".
How to do this in XP
Sleep and hibernation: frequently asked questions