two internet connection

my laptop has two internet connection .  WI-FI internet and a USB Modem internet connection.

I want to give WI-FI internet as highest priority . If WI-FI  internet disconnects , then USB Modem internet connection will be utilized . Also, whenever the WI-FI comes back , USB Modem should be idle.
How to achieve this ? I have windows XP OS.

what is the solution for this ?

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see here:
How to change the interface metric on a network adapter
i think it is not possible to do in real time to switch between connections.but you can set the priority which to use first.

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What you are looking for is kind of like NIC Teaming.
The only difference is that with teaming both cards care connected, to ensure there is no gap in connectivity if one fails.
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cofactorAuthor Commented:
I want my system to be Always connected.
I want to use  WF-FI as first priority . But If WI-FI goes outage , then I want the connection from USB Modem to ensure that there is no gap in connectivity if one fails .However, If WI-FI comes back then I wanna take the connection from WI-FI again ....because my WI-FI connection is  FREE ....but USB connection is chargable :)  
What should I do ? please help
if you use some software router or firewalls, you can have more control on your connections.
Are you talking about outgoing?  Or are you running some type of server that you need to change the incoming stream from wireless to DSL?
It might help to understand what "always on means"
Just how unreliable is your wireless connection?  If it is dropping constantly, you should fix that first.  Or by free wifi, are you conneting you a neighbor?
cofactorAuthor Commented:

This is for HP laptops . this is HP utility . I have acer laptop hence I can not do it.
cofactorAuthor Commented:
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