If you have physical access ... boot off a Linux distro CD like knoppix and run shred on the hard drive device /dev/hda or whatever.
If remote... you can shred individual files, but that still leaves information on unused disk blocks. So..one possibility is to remove as much as you using shred on individual files, then create a large file
dd if=/dev/zero of=/garbage bs=$((1024*1024))
then shred the /garbage file. The /garbage file will have zeros in it initially, but if someone really wanted to they could recover the information that was stored on it before.... If you're paranoid, shred as well.