IPhone 3 GS OS 4.2.1 is not able to update

When I am connecting to iTunes, iTune is not able to find any updates hence I have downloaded the update file and tried to update with the downloaded file. But an error message is displayed which says that ‘It is not compatible’. Also, there is nothing shown after the message. I am sure about the downloaded update file as it is for 3 G S only.
I have also checked in the forum, but there is no information available. I am using HP 6830 which runs with Win – Vista and it has completed all updates.
Please help me to get out of this problem.
I will be thankful for all the help.
