Try this:
# Run
AppInstaller and select
Firmware 1.0.2 AppInstaller will download and restore the firmware for you.
OR
# Open iTunes, Goto Recovery mode by holding
Power + Home on the phone
# iTunes will also detect 'Recovery mode'
# Download ths iPhone 1.0.2 firmware
Code:
http://appldnld.apple.com.edgesuite.net/content.info.apple.com/iPhone/061-3823.20070821.vormd/iPhone1,1_1.0.2_1C28_Restore.ipsw
# Press SHIFT + Restore button on iTunes and manually select the firmware you just downloaded.
Hope this helps!