You can leave it *on* for months. Hibernation uses zero power. What it does is it puts everything in the RAM onto your hard drive, so when you boot up again it just loads it back into your RAM. I recommend restarting weekly regardless because with Hibernation, Windows doesn't quite clean up after itself and your RAM will be used more and more each time. But yes, use hibernation over shutting it down.
You also might want to check out some tweaks for making the WinXP boot faster, the first of which is to remove junk from your start up. 'Start' -> 'Run' -> 'msconfig' -> 'startup pane' and have fun! You normally don’t need anything other then your keyboard and mice drivers, if they have extended options (Like my Logitech keyboard has a bunch of extra buttons, and a driver called ‘iTouch’ which I have auto startup.)