It is extremely easy to do in Windows XP.
All you need to do is to purchase 2 USB wireless adaptors (1 for each computer). Plug them in, let XP install the drivers and then run the network wizard in XP on the machine that is connected to the ADSL.
Follow the wizard all of the way through and you will have a wireless network.
As far as adding another computer at a later date, that will be simple too. Simply purchase another USB wireless adaptor, add it to the new machine and insert the network floppy disk that was created the first time you ran the network wizard.