Powering down the display in XP with a command

There is also a utility called NirCmd, which supports not only this but more commands (like changing system volume, manipulating services etc - documented in the page linked). Usage is via the command line:

nircmd monitor off

And sometimes I've seen Windows "listening" to your last keystroke (the Return that confirms the command) after the command is "issued" and waking back up. A solution that has worked for me in most cases is:

nircmd cmdwait 1000 monitor off

Disclaimer: although this may be the third or fourth time I mention it here, I have no association to the NirCmd's team: just a happy user.

Not as far as I know but you can use utilities like Monitor Off to power down the monitor.