First of all - this sounds like a bit of a security issue. The machine could begin acting as a router if it was compromised, which would then compromise the 192.168 network. Hopefully this is not your office LAN environment. It would be better to establish a firewall connected to both Internet connections (as untrusted connections) - with your web server in a DMZ network. Something like... (pardon the ASCII art):
ISP A -> firewall < - ISP B
web server everything else
You could set up port forwarding on the firewall so that connections to port 80 of either ISP's IP forward to your web server, which is on a physically separate DMZ network. Now, your budget / network architecture etc. might prohibit this.
If that's the case; a simple route add should do the trick. The help file from route print:
> route ADD 188.8.131.52 MASK 255.0.0.0 184.108.40.206 METRIC 3 IF 2
destination^ ^mask ^gateway metric^ ^
If IF is not given, it tries to find the best interface for a given
Is less than terse, but correct. Assuming you want this to be a second route to the outside world, the syntax for the destination would be 0.0.0.0 mask 0.0.0.0. For the gateway, this is the IP of the machine that gives you access to the second ISP. I usually skip the metric and IF part, but I think you probably want to set the metric of your "main" ISP to 1 and the metric of the "backup" to 2.
That being said, I'm not totally sure what Windows will do if you enter this. It will be able to use both gateways, and I think what it will do is prefer the one with metric 1, but if that fails use the one with metric 2.
More info available here: http://technet.microsoft.com/en-us/library/cc779696.aspx