Keygrip
Does it get to the BIOS screen and, if so, what happens?
If you get past the BIOS screen, I would personally boot from a Winblows CD, choose the "Repair from console" (or whatever it says, R is the key to hit) option and then "fixmbr".
If you don't get to the BIOS screen, then the first port of call is resetting the BIOS. If you don't know how then tell me the make/model of the motherboard or computer and I will sniff out the manual for you. Usually it's just a jumper that has to be moved, but sometimes you have to merely remove the CMOS battery, wait a minute, and then replace the battery.
If that don't work, make sure you tell us the details and I'm pretty sure we'll figure it out.