It is not enough to have a DHCP and a TFTP server. Your computers won't be erased while booting on the network, if you don't setup the BIOS of each computer you would like to erase.
Make your computer boot from the network by searching for an option that mentions about booting from the LAN (the name and the location of this option depends on your BIOS version). In some BIOS versions, you may find this under "Advanced BIOS" or "Boot sequence".
After doing this, when each computer is restarted, east-tec DisposeSecure will start running and erasing your computers.
If you have a large number of computers on your network, and you don't want to go to each computer to configure the BIOS to boot from the network, you could use a special bootable CD. Read more