Do one of the following:
- Click Start->Turn off Computer->Turn Off
- If that doesn't work (for example, everything is locked up) then press the power button.
- If it won't turn off when you press the button, hold the button in until it does go off.
- If it still won't turn off, pull the power cord and (after 10 seconds) plug it back in.
Now, press the power button once again, and it should come back on.
This should reboot your computer. (All it really means is turning your computer off and back on)
Hope this helps,
WallyH