Why not just re-install Vista?
If it came on your machine when you bought it, you should have the disc, and the serial number, so there is another option.
If you are going to say, like most people, I dont have the disc, then go back to where you bought the computer, and ask for it, because if it is on your computer, then you have paid for a license for the OS in the price of the computer, and it is illegal for them to not supply you with the disc and serial.