When implementing DHCP server it is necessary to send Server Identifier option field in the DHCPOFFER message otherwise Windows XP DHCP client will not work fine.
As RFC states you must use
Subnet mask (DHCP option code 1) Router address (DHCP option code 3) DNS servers (DHCP option code 6) Server Identifier (DHCP option code 54)
but if you don’t include Server Identifier, the host machine is still able to configure succesfully but Windows will report connection problems.
As allways the best advice here is to follow RFC strictly especially about mandatory fields.