How do I write to zero, on my hard drive? thanks
Why do you want to do that?
I think that there is a virus that reformatting is not enough.
Trust me, reformatting should be enough. There haven't been any bootsector-type viruses for a number of years now.
What symptoms is your PC exhibiting? If you have a problem that's persisting, even after reloading Windows, then I'd start suspecting some type of hardware problem. You might want to explore that avenue instead of trying to figure out how to zero-out a hard drive.
If you want to be sure everything's gone, learn how to use FDISK and delete the partitions and recreate them. That will leave the drive about as clean as you would ever need to get it.
Unplug your network cable when you reinstall, and before you do anything else put up a firewall. Then, install antivirus software. (You can download Zonealarm for the firewall, and someone will have to suggest antivirus software.) Even 20 minutes without a firewall on a LAN could mean getting a virus. (Especially with a default Windows install.)