Windows 7 is required on the notebook acting as a wireless hotspot, but any wireless-equipped device, including PCs running Windows XP or Vista, or even Mac laptops, can reach the Web through Connectify without any additional software. Connectify also encrypts the traffic to and from the software "hot spot" using WPA2-Personal (AES) encryption.
The can be downloaded from Nomadio's Web site.
Apple's Mac OS X already offers a similar feature under the "Internet Sharing" preferences setting.