Are you able to see the manufacter's splash logo before failing to boot, or does nothing come on screen?
My parents' old Toshiba Satellite had a similar problem, where I would press the on button but it wouldn't boot after 3 attempts and shut down. I opened her up and found that the cable connector from the power button board to the mobo was fried. If you have time search how to open your Dad's laptop and check for discoloration or shorted out items. Replaced the cheap cable and it booted just fine. And always take out the battery and hold the power button down to discharge as much as possible after removing the battery.
If you do not have any sensitive material on that laptop I would first flash the bios. If that doesn't help grab the Vista install CD. If the CD is not availible download the Vista .iso and mount it on a CD. And do a clean install of Vista. If you do need recovery of the data I would remove the HD and find recovery services.
If you have questions on how to do this, ask and provide the make and model of your laptop.