I have 2 Internet connections on my computer: One Broadband cable, and the other a dial-up line. Both are active at once. I run Windows 2000 and have .cmd scripts (triggered via DOS icons) that set my route to one or the other. If my broadband goes down, I can switch to the other one very quickly by executing my dial-up script, which sets my default route to the dial-up IP.
Sample code for this script (here's the one to set the default gateway to my broadband gateway):
rem
rem 205.41.162.37 = My dial-up IP
rem 192.168.2.42 = My broadband wireless router IP
rem 205.41.162.254 = Dial-up Gateway Address
rem 192.168.2.0 = My broadband wireless router gateway Addr
rem
route delete 0.0.0.0
route add 0.0.0.0 mask 0.0.0.0 205.41.162.37
route add 192.168.2.0 mask 255.255.255.0 192.168.2.42
route add 207.41.164.0 mask 255.255.255.0 207.41.164.37
route add 205.41.162.254 mask 255.255.255.255 205.41.162.37
Sample code for this script (here's the one to set the default gateway to my dial-up gateway):
rem
rem 205.41.162.37 = My dial-up IP
rem
route delete 0.0.0.0
route add 0.0.0.0 mask 0.0.0.0 205.41.162.37 metric 1