A couple of things:
1. I think it is easier to use QuickPwn instead of WinPwn now to jailbreak the 3G on a PC. I used WinPwn the first time mainly because they came out with the ability to jailbreak on Firmware 2.02 first. Now, however, QuickPwn allows you to jailbreak with the latest firmware update (2.1). Plus, it is easier - you simply upgrade the firmware using iTunes, then run QuickPwn. It isn't necessary to build an IPSW file like in WinPwn.
2. To use QuickPwn to jailbreak an iPhone using firmware 2.1, follow these instructions
a. Use iTunes Version 8 to upgrade to firmware 2.1
b. Download Firmware 2.1 seperately
c. Download QuickPwn
d. Put QuickPwn and Firmware 2.1 in the same folder
e. Run QuickPwn and Follow the instructions
*quickpwn doesn't upgrade your firmware, it just jailbreaks it.
1. iDevTeam blog post about 2.1