There are lots of ways your old network could have been set up.
Was the old XP machine connected to the router by LAN or USB cable.
Did you need the old XP machine to be turned on in order to connect to the internet from your phone?
What I'm trying to get at is was if the XP machine set up to "share internet connection" and whether you can change that so you directly connect to the router instead.
What make/model router is it anyway?
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If you could connect to the internet even when the old XP PC was turned off I'd reboot the router.
If you couldn't we need to make sure the router has wireless capabilities, and if so work out whether it can provide wireless networking to all your devices.
If it has no wireless capabilities you are going to need to do something lie this:
http://lifehacker.com/5369381/turn-your-windows-7-pc-into-a-wireless-hotspotbut I'm not sure I'm the right person to advise on this as I've never done it - happy to play on my own PC but less so in a thread on the internet!