im current owning an Iphone 3G 8GB sim free US set,,,,.. bought as 2nd hand phone from forumer

the forumer help me to installed to is 4.0.1 and JB it..
however, there's some problem with the location service and i don seem getting my gps software works.
therefore i decided to downgrade it back to 3.1.3

BUT, the problem is here.i not able to downgrade it as im using window.. (realize after i flash my iphone)
right NOW...i stuck at the DFU screen

so my question is.. if i dl the 4.0.1 firmware and use it to restore back..
will it works?

need help here seriously.. I CANT USE MY IPHONE NOW

ANY CAN HELP!! ??!!

current owning a iphone 3g 8g SG set officially unlock
wana upgrade to os4.0 but dono the method...
is there any1 there can help me?