Configuring ISP Backup/Redundancy

I need to find a way to setup seemless internet connectivity. In other words,
if my primary internet connection were to become disconnected for any reason or period of time, I need a backup connection to the internet to instantaneously kick in.

At present I am using Windows XP, Comcast Broadband and a router for my network. I imagine the backup would most likely be Quest DSL.

Any help would be greatly appreciated ;-)