Download and create Hirem's Boot CD.
http/www.hiren.info/pages/bootcd
Boot off the CD and select one of the disk tools which allows secure erase, like DiskWipe or Active Kill Disk. Use it to secure erase the drive (fill it completely with zeros or random data).
A regular delete or format just changes one byte at the beginning telling the OS that this file/partition is erased. You can get the data back by changing this one byte back. A secure erase overwrites all the data, making it unrecoverable unless you use military-grade recovery techniques (i.e. taking the drive apart, reading it for trace magnetic signals).