Hey,
I did the same thing, before I sold it I bought a commercial disk wiping program from Office Depot for about $20 - it overwrites every bit on the drive and zeros out everything. I've also used it on a system that was unrepairable due to viruses, after wiping the disk and using the restore media provided by the manufacturer it worked fine. (That time - I currently have a Vista laptop where the same thing happened - but this time the bios is hosed and the machine won't even recognize the hard disk correctly - it reads it as a removable disk - go figure. Have to return it to HP for this one, but WipeDrive from WhiteCanyon will remove ALL data from your disk)