I have BT internet with an ADSL router and need to have wireless so that I can get internet in another room.
What equipment do I need for:
a) my desktop.
b)my laptop, which has a built in wireless connection.

Must I also change the ADSL and get a WLAN?

Hi there. Firstly, let me clarify something for you: WLAN means Wireless Local Area Network. "Local", in your case, meaning that it's just communication between the PCs in your house. The ADSL Internet connection is the bit that goes outside of your house and communicates with the rest of the world. So you actually want both: ADSL to get out onto the Internet and a WLAN to get onto the ADSL link.

You basically need to replace the little USB modem that was supplied by BT Internet and replace it with a wireless ADSL router. This is a modem that plugs into your BT phone socket still but it also has an aerial (rather than a USB cable) to communicate with your PCs over the air via wireless LAN. Typically, wireless routers also normally come with 4 RJ45 ethernet ports so that you can connect more PCs via LAN (standard cabled networking) but you will probably never use that feature.

If your laptop has a WLAN connection as you say, then you don't need to buy anything more for that.

Your desktop PC will need a wireless network (WLAN) card. These come in two forms. The most common is a PCI card that you have to fit inside the PC into a spare PCI expansion slot on the motherboard, if you have one free. However, a much easier method is to buy a USB WLAN "dongle", which is basically a wireless network card that just plugs into a USB port in the back of the PC so you don't have to worry about messing about with the insides of it. USB adaptors cost a few pounds more but there's not much difference in it for the benefit that you get. It also makes it incredibly easy to transfer to a different desktop PC if you want to later on. It's certainly what I'll be getting when I switch my BT broadband to wireless.

So that's all the hardware you need. Easy really. Configuring the software and network connections is a different matter completely .... but you should receive instructions (and maybe even a really straightforward Setup program) with the router and/or network card.

Hope that helps