Do you want to have multiple gateway entries on your PC or do you have a network with multiple gateways or routers on it? Please give us a little more information and we'll try to help you
Look into using HSRP. It creates a virtual gateway that is shared between 2 routers, making one active and one stand by. if the active router dies, the standby grabs the virtual address and becomes the active gateway. The routers can also be active for one network, and standby for another network in case different segments need to use a specific router under normal circumstances.
At the end of the day, a user is not going to mess with plugging/unplugging cables when a router goes down. They are going to call you and waste your time, so make it easy and add the functionality to the router.
d_jabsd is right. You could achieve the same with several static routes carefully thought out and applied, but why do things the hard way?
One thing to mention, HSRP is the Cisco method of providing router redundancy. Some other manufacturers use VRRP which is very similar and does the same job
I haven't tried this personally, but I believe you can add aditional gateways via the 'route' and/or 'netsh' utilities on Windows 2k/XP or greater. Play around with them and see what you find. A good place to start is 'route print' to display the current routes your system knows.