Cisco router as gateway, XP SP2 client updates routes

Hello all,
We were running a CISCO 2600 as a gateway on our network of Win2k3
and XPsp2 machines, and it appears that when a client was trying to get
to a specific location through the gateway as expected, the client
"learned" the route so the next time the client needed to get to the
resource, it used the correct gateway (not the cisco in this case).
I'm aware of silent RIP in win XP, but none of our machines use it.

Does anyone know by what mechanism the XP clients get this route
information and auto-update their own tables?

We had recently switched our default gateway to another device, and now
the clients seem to always go through the default gateway, even when
the client has found the resource before.

any ideas would be great!

thanks!