managed switch and dhcp addresses

Hi all,
Does anyone know of problems using a managed 3com switch not receiving dhcp
addresses relating to ISA server firewall client?
It seems that when the isa server firewall cllent is installed then the
managed switch cannot get a dhcp address from the network.
my setup is:
ISA server and firewall client installed on workstations
server 2000 running dhcp
server 2003 running dhcp
(purpose for failover)
scope 192.168.1.50 to 192.168.1.240
3com switch assigned static ip of 192.168.1.49
when I plug the new 2250 switch into network then the workstations revert
back to a 169. number and cannot get a dhcp address.
Thanks in advance for any help on this.
Sher