I'd recommend you don't buy from PC world. With the exception of some special offers, they're expensive. They also have the worst staff of any computer shop I've ever been into.
Lots of computer shops, personally I like ebuyer for this sort of thing. Brilliant service and delivery, and good prices.
Assuming that your current modem for your internet connects to your computer upstairs via ethernet (the cable that's got a connector a bit like a telephone connection), then you want something like this:
http://www.ebuyer.com/UK/product/48546
That's also wireless, so not only with both your laptop and PC be able to get online at the same time, you'll also be able to use your laptop all over your house online, and probably even your garden. (You laptop probably already has wireless capability.)