[Stephen]

Hi and welcome to the site.

Wireless N also does Wireless B and G so will work fine with your existing router. I have the Virgin Media Netgear router and an Intel Wireless N card in my laptop and they work great.

You won't have any problems running both Vista and XP.

As you are using a router your main PC will not have to be on as the laptop will send and recieve its signal over the wireless and has no need for the PC to be involved.