I have a home office with three main PCs and a laptop, which I want to use anywhere in the house.
All the PCs have a USB wireless adapter (Belkin) and you will have to provide adapters for your computers. You can get a card that fits inside your PC if you prefer that, but the USB external ones are just as good and far quicker to install.
The laptop has a wireless adapter that plugs into the card slot (Linksys, and you couldn't hope for a neater solution - it works perfectly.
I have a Linksys 2.4 Ghz wireless access point.
And finally an Alcatel USB broadband modem, connected to the BT business ADSL service via one of the PCs. All the computers are running WindowsXP Professional.
I connected everything as per the manufacturers' instructions and set up a network using the Windows XP networking wizard (run it on each machine).
It all worked first time, and has continued to do so. The only glitch occurred when the Outpost firewall that's installed on the Internet-connected machine refused to allow the other computers any access to the broadband connection. A quick tweak of the settings put that right.
It really is that easy. If you want to use a router you can. There are several that will accept a USB ADSL modem, and in fact there are several that have their own ADSL modem built in.