I believe Untangle will do it. it has two apps your can install a WAN balancer and a WAN fail over. But you need to pay a monthly fee for them both
but this will replace your server 2003 OS with Untangle OS, so I am not sure if that is what your are looking for. Unless you have a secondary computer you are not using your can use.
I think there is a Untangle you install on top of windows but that also cost money, and I am not sure what it does.