Debian uses /etc/network/interfaces to configure network interfaces to request an IP address from a DHCP server with a line like:
iface eth0 inet dhcp
However if you are using a PCMCIA card as your NIC you should also check your /etc/pcmcia/network.opts file and ensure that there is a line DHCP="y" and that there is no setting for a static IP address.
A static IP address setting is likely to be found in this file if the Laptop has been setup to use a static IP address prior to wanting to use a DHCP server. The /etc/pcmcia/network.opts file overrides any setting in /etc/network/interfaces so you will end up with whatever static IP address is in this file. This can be reset temporarily by running /etc/init.d/networking when you should find the card will then pickup an IP address via DHCP.