I'd go to the hard drive manufacturer's site and download a hard drive toolkit -- which will include a program which can run from a floppy disk to write zeros all over the drive.
Bear in mind that this will wipe any partitions including ones that may have been put on by the computer's builder to carry the operating system backup image in case you needed to reinstall Windows.
You can use a software called HDShredder. Is very good and very easy to work with. You have to make it bootable on a CD or flash drive. And then you boot from it, you follow the guide (basically you only press next because the default values are the best) and then the process begins. Is wipes the sectors on the hard drive and restores the disk's integrity. After it finishes you have to insert the windows installation disk and create partitions etc.