While Bzeek offers an excellent free utility to share your internet connection over WiFi, another free and open source software based router for computers using Windows 7/Windows Server 2008 R2 is "Virtual Router".
Virtual Router is a software that enables users to wirelessly share internet connection on their PC (Wifi, LAN, Cable Modem, Dial-up, Cellular, etc.) with any Wifi device (Laptop, Smart Phone, iPod Touch, iPhone, Android Phone, Zune, Netbook, wireless printer, etc.) without requiring WiFi router hardware. The utility creates an "Virtual Router" by utlizing Windows 7 built-in adhoc WiFi sharing capabilities - the acess-point created is completely secured using WPA2.
Virtual Router from CodePlex