You can put your computer in any of several different power-saving states, depending on whether you value your time or your electricity bill.
Know your power modes: Windows PCs support six different "power states": S0 (fully running), S1 (CPU stopped, RAM refreshed, running in low power mode), S2 (CPU off, RAM refreshed, running in a lower power mode than S1), S3 (traditional Standby, with the CPU off and RAM in slow refresh), S4 (Hibernate, with the hardware off and system memory saved as a temporary file), and S5 (Off). Note that S2 sleep mode is often disabled in computers and is accessed only when the computer is unable to go into S3 sleep mode.
Decide between Sleep and Hibernate: S3 is typically the default Standby/Sleep mode, while S4 is usually the default Hibernate mode. To access S3 or S4, simply click Start and then click the triangle next to the Shut Down button.
You can choose Sleep (Standby) or Hibernate from the menu that pops up. Resuming a computer from S3 sleep mode normally takes a few seconds, whereas resuming a computer from S4 sleep mode takes a few minutes, so use S3 for shorter sleep intervals and S4 for longer ones.
Speed up sleep with S1 sleep mode: If you're looking for an option that will save power and resume even faster than S3 sleep mode does, you can access S1 sleep mode. Resuming from S1 usually takes less than 2 seconds. To access S1, open your system's BIOS by restarting the computer and pressing the appropriate key during boot-up—typically you'll see a message similar to 'Press Del to enter Setup'.
Once you are in the BIOS, go to Power Management, Suspend Mode. There you will see an option to change the default suspend mode from S3 to S1. Change it and press Esc to exit the BIOS (be sure to select the Save option when it prompts you to save the settings).
From now on, when you choose Sleep from the Start menu, your computer will go into S1 sleep mode. Just a note for you conservationists out there: S1 sleep mode uses about 112 watts to S3's 5 watts, so it isn't as easy on your electric bill (or for laptop users, your battery) as S3 is.