The perfect home network, in my estimation, consists of a cable or DSL broadband connection, an AirPort Extreme Base Station, an AirPort card in your Mac mini, and a wireless device for each of your Windows PCs. Add AirPort Express Base Stations as desired.
Ideally, your Windows computer has USB 2.0 ports, which are fast enough to support an 802.11g wireless adapter. If your computer is older and has USB 1.1 ports, you can buy a less-expensive 802.11b USB wireless adapter. It runs at 11 Mbps, about the same speed as the older USB ports.