In general, for each ISP you will (or should) have a separate router. These routers will each provide a DHCP service where they assign IP-addresses to computers connected to them.
Each router should be configured to have a separate pool of addresses in it's DHCP pool. For example one router might have a pool of 192.168.1.2 through 192.168.1.254 with a subnet mask of 255.255.255.0. In that case ensure the other router is using a separate range and a separate subnet such as 192.168.2.2 through 192.168.2.254 with a subnet mask of 255.255.255.0.
If this is done, the computer should pick up the correct addresses and have no local routing conflict. I am unsure what algorithm Windows Server 2003 applies to routing to other networks when the gateway routers have the same routing cost (e.g. hop-count). You will probably find that one ISP is favoured but if that ISP is down I would expect that the operating system will eventually mark the route down and redirect traffic to the other ISP. This is speculation on my part.