Laptop's usually use a momentary switch to power on so here's what I would do (not that I'd suggest it if you're not experienced in things like this);
Disassemble your laptop and look for the power button, now short each wire/solder point on the button with every other wire or solder point, shorting 2 at a time until your laptop boots up (there's usually only 4 wire/solder points)
Once you know which 2 points have to be shorted to start the laptop you'd solder 2 lengthy wires to those points and a momentary switch (eg an arcade button!) to those lengthy wires,
Some modification will have to be made to the laptop casing if your going to reassemble the body because you can't screw it back up with 2 wires and an arcade button hanging out.
The momentary switch would then be mounted on your front panel, or somewhere it won't be accidentally pushed and further to this windows has a setting that will turn off the laptop when the power button is pushed again.
Hope that helps (and I of course take no responsibility if any goes wrong, be careful!)
(edit) Oops it seems I should have read the link in TPA5's post, it's what I was trying to describe