Thanks you for answering so fastly.
Before all, i want to precise that i don't use router.
I have a windows xp professionnel edition pack 2 on my computer.
In one part, i have a internet connection by the lan network(so i use a network card). On another way, i install a dial up connection(CDMA) which provide me internet too(so I use a serial port com).
The problem i have, is how to make the lan connection Internet be primary connection and and the dial up connection Internet be secondary connection; So when the first fails the second automatically provide internet and when the lan connection Internet is up, the secondary connection is disabled automatically to permit the primary connection provide the internet.
I hope my details will help you.