Wifi card needed

Well it's always best to use a wired connection on a desktop PC if your router is near enough to be able to do that. Not only will it be a much more reliable connection, but you'll also need a wired connection anyway if you ever need to re-configure the router's wireless settings using it's web-based configuration utility.

However, if you must have wifi on your PC you can either install a PCI wireless card or get a USB one. A PCI wireless card means you can leave your USB ports available for other devices instead (though that may not be an issue if you have plenty of USB ports anyway).

With a PCI wireless card it should be "plug & play" with most of them. You just install it then fire up your PC. Windows should automatically recognise it and install an appropriate driver that's built in to Windows, otherwise you install the driver supplied with the card.

With a USB wireless adapter, you install it's driver before plugging in the USB device. The installation utility will tell you when to plug in your USB device, then the software installation continues.