First, why use such an old version? Unless you have a GREAT reason to go that way, I highly recommend that you get version 6.2 or above. It'll be much faster on bootup, and quite possibly the DHCP thing will become a non-issue. Your router is supposed to provide the IP address and such with DHCP, but it could be faulty. In any case, you should probably burn again (either with your current ISO or a new one for 6.2), or at least run the media self-test (press F2/F3 for options when you get the first "big-logo" screen).

Cheers!
Krishna