Hello,
I have a 3G iphone running 3.1.2 firmware with modem 2.28. The phone has some problems ( network issues ) and i need to resinstal de firmware but I dont want to upgrade it to a newer modem/bb version ( or at least not to a modem version wich is not suported for unlock for the moment ). Wich is the safest way to do that ? I only have a windows computer so I cant use mac options. The phone is already jailbreaked and it was working fine till yesterday. I need to reinstall de firmware and , after that, jailbreaking and unlocking it back.
Need answer A.S.A.P.
Thanks in advance.
B.R.

you should not be running 3.1.2 with 02.28.00 BB at all . That 02.28.00 BB is from the 2.2.x firmware day (yellowsn0w).

You should be running 3.x with 04.26.08 BB and use ultrasn0w.
If you want to use 02.28.00 then you should be using firmware 2.x

Do this: (follow the instruction).

- restore your iphone 3G with Apple 3.0 firmware. This will update the BB to 04.26.08. This is a supported BB for ultrasn0w unlock

- jailbreak by using redsnow.
- install ultrasn0w
Now your phone is jailbroken & pwned & unlock.

FROM this point on, ONLY USE CUSTOM Firmware created by using pwnagtool 3.1.4. DON'T use apple original firmware 3.1.x.
Using custom firmware is the only way to preserved your BB at 04.26.08.

While jailbroken at 3.0 fw, you can simply shift-click restore and select the custom FW. when done, reinstall ultrasnow