Windows 7
You can create an ad-hoc network or download
Connectify software
into your machine. In my case, I am using Google Nexus One (rooted)
with 3G network. My Laptop connect to my mobile network then share to
the others through Connectify. Just follow the
Easy Setup Wizard to configure.
If you are using another different OS (XP, Linux, Mac) without
supporting of Connectify, you can create ad-hoc network for sharing
(just google it) or using RJ45 cable.
There is another option which I am using on my desktop. I am using a
Desktop in my house and I want every times it turn on, It will
automatically open the WiFi sharing for my house. I can buy a WiFi modem
or another WiFi usb. They both work well. My recommendation is a Ralink
USB WiFi with Ralink ultility to turn Usb into Access Point.