The first step in the process is called "tethering" (meaning you connect your computer to the Internet via your phone's Internet connection), and some phones and carriers support it. I don't know of a comprehensive list of carriers and devices; you'll have to check with your carrier to see if they provide the service, check your plan to make sure you're allowed to do it (otherwise you'll incur massive data charges), check to make sure your device can do it, and make sure the software is available for your OS (although sometimes you can use either the built-in Windows dial-up utility or connect via Bluetooth, depending on your hardware).
Now, if you can tether your device, it only takes a little more effort to turn your computer into a wireless access point. Follow the steps in these (or similar) articles:
http://support.microsoft.com/kb/306126
http://www.practicallynetworked.com/sharing/xp_ics/