Restoring with custom firmware

I realise I am probably being incredibly thick-headed, but I hope you people can help me out...

I have an iphone 3g, and I am trying to unlock it as I have moved out of the US. The phone currently has a baseband of 2.28.00, and is firmware 2.2 (5G77)

Using the pwnage tool, I was able to make a custom firmware, but kept getting a 2001 error. According to other posts, this seems to be a USB issue on my MAC. So, I decided to save the custom firmware on to a windows XP box, and put the phone into DFU mode to restore there.

The phone successfully goes into DFU mode on the Windows box. However, if I click 'OK' to allow a restore, iTunes wants to download a new restore image, and install that. I do not think this would be a good idea to allow, since it will not be using the custom firmware that the pwnage tool created.

In short, my question is how do I get iTunes to install the custom firmware instead of downloading new firmware?

Thanks in advance for your help!

If your baseband is 02.28.00 you can unlock it right away with YellowSn0w. No need to use custom FW.

Regarding to your question, in Windows press shift + restore. In Mac, option + restore and you'll be able to choose

Thank you for trying to help me.

I was able to restore as you suggested, however, it errors out on an error 21.

I tried a powered USB, but it does not seem to do the job.

You mentioned that I should try just using yellowsnow, but I do not think I can do this unless I use the pwnage tool, as I do not have cydia on it yet. Additionally, the phone now is only in retore mode, and I have to restore it with something, but it fails.