DHCP Not holding setting

I have a Standard Windows 2003 Server network with XP Pro clients. THe
DC has DHCP setup with a scope assigning IP addresses within a range of
x.x.x.100 to x.x.x.200. I have also set in the DHCP scope the Gateway
of x.x.x.254 and the PC's get this no problem at all.
The problem is we have 2 Cisco routers in that one office with separate
links to the ISP. The Comms Engineer has these set to failover to one
or the other in the event of an outage or router failure. The problem
he is getting is the clients are not keeping their DHCP assigned
Gateway.
When he sets the gateway on one to x.x.x.250 for example the PC's all
change to that automatically even though the DHCP Scope still remains
at x.x.x.254
This is effecting the failover because the clients need to stay at
x.x.x.254 and not automatically find a new gateway when it appears.

Any ideas?

Further to this it appears the Routers are configured using Cisco's
HSRP in order to give redundancy should one router fail the other
router will take the gateway address. This fails over OK but the
clients don't seem to stay looking for the default gateway set to them
by DHCP.