I've found that the shut down and sleep options are right there in the Windows orb (aka "start button"). If you modify the power settings you can probably also enable/disable hibernation functions too (if it isn't already).
But really. You may have to enable the S1 or S3 power configuration in your BIOS in order for sleep to properly work. There may be other settings too! You may even need to check your power supplies ability since it is possible to actually shut down even though you're actually trying to put the system to sleep. And the reason that might happen is because there might not be enough current on the 5V rail which is necessary to keep the RAM and RAM circuitry alive in a true sleep condition. (Now you know why it's so dangerous to plug/unplug expansion cards or add/remove RAM modules when power cord is still plugged in.)
Otherwise, hibernation and possibly enabling WOL (Wake On LAN) might work? But WOL would require a router capable of handling those "special" packets. Hibernation also doesn't allow a system to wake on a particular event such as if you were scheduling WMC to record a television show or even if you wanted to schedule Windows to defrag itself at 3:00AM. Hibernation is really only good for booting fast. You'll still go through the BIOS POSTing and all, but then Windows will appear to almost instantly turn on since the RAM is essentially reloaded from a huge temporary file on the HDD (this is really more for laptops when you think about it).
Anyway, good luck. I think the reply from "superuser" is more than adequate for your immediate question of what command to use. Although, I might have just created a shortcut and possibly used this for the command: Rundll32.exe Powrprof.dll,SetSuspendState Sleep (Here's a link where I think I saw it: http://answers.microsoft.com/en-us/windows/forum/windows_vista-desktop/is-there-any-way-to-make-a-desktop-shortcut-for/4e57f536-50a8-4942-a84a-290b3dea1ffb )