using the linux program dhclient.
If you don't have linux installed, you can use a bootable Linux live CD.
 If you do this trick to create an address reservation for an actual Windows machine, first switch the NIC in Windows to a static IP, because after a reboot in DHCP mode Windows may send a DHCP request with its last used IP, which would annihilate your efforts with dhclient below...
The trick is to send a DHCP request with your desired IP address from the same NIC (or better: same MAC address).
Edit the /etc/dhcp/dhclient.conf and add the following line (you might have to first copy the file to a writeable location if booting from a CD):
send dhcp-requested-address 192.168.1.240;
Then stop and start dhclient as user
root, where your NIC is
eth0 (check by issuing
ip addr show beforehand)
dhclient -r -v
dhclient -4 -d -v -cf /etc/dhcp/dhclient.conf eth0
If successfull, your DHCP server will fulfill your prepared request. Sometimes you will have to delete the device entry in the DHCP table before issuing
dhclient -4 -d ....
Then make that device IP address in the DHCP table of the router a fix address reservation. This will associate the IP to the MAC of the device and return the same IP everytime a DHCP request with that MAC address is encountered in the future.