Two ISA servers and what if one goes down..Redundancy? Please help

Hello,

I'm running two ISA 2006 servers in my network for internet access and running as SecureNAT client. How can I automatically route traffic of one ISA server to another ISA server if one ISA goes down? I mean without going to client side and changing their default gateway IP to second ISA which is UP.

Here is how it is configured:

Two ISA Servers:
ISA1 IP: 192.168.1.4 (ISP 1)
ISA2 IP: 192.168.1.5 (ISP 2)

User Workstation:
IP: 192.168.1.10
Default Gateway: 192.168.1.4 (IP of ISA1)

What if ISP 1 (ISA1) goes down then how can I let users to route their internet traffic to ISP 2 (ISA2)?

There are around 50 clients in my network and their LAN adapter is configured with static IPs (no DHCP).