As best I can tell, there are lots of options, but they are all essentially the same thing. And every one seems to require about 3-4 manual steps.
1. Have the PC create an ad-hoc WiFi network.
2. Tell the iPhone to join the PC's ad-hoc network.
3. Run something on the iPhone that acts as a SOCKS Proxy
4. Configure your PC's web browser (firefox) to use the SOCKS Proxy on the iPhone
Step 3 appears to have about 3 choices:
1. 3Proxy is a Socks app in Cydia. You can launch it from a command line on the iPhone using a terminal app, or you can use BossPrefs to create a button that will start & stop it. 3Proxy and BossPrefs are both free.
2. iPhoneModem in Cydia appears to just put a GUI in front of a Socks proxy, this is shareware and they are asking $10 to register.
3. there is a 2nd application in Cydia that is also named iPhoneModem, it appears to put a GUI in front of a Socks proxy, but also includes 'insomnia' to keep the network alive when the phone screen is off. This one is free.
If you can complete these steps manually, then nothing else is needed on the PC side. I managed to get this working the other day, and should be able to set it back up in a few minutes.
Edit: I found this document useful, as I was missing a setting in firefox trying to get it to use the socks proxy correctly:
http://www.iphonemodem.com/firefoxGuide.pdf