Connect your iPhone to the computer and put it into DFU mode. If you don't know how to do this you can find instructions here.
Press to launch Control Panel from your Start Menu.
Press to select Hardware and Sound from your Control Panel window.
Press to select Device Manager from the Hardware and Sound window.
Double click to expand the Universal Serial Bus Controllers category
Right click the Apple Mobile Device USB Driver item and click the Properties button.
Click to select the Details tab.
Select Device Instance Path from the Properties dropdown.
Scroll to the end of the field to locate your bootrom version. It should look something like [iBoot-359.3]
***If you have an iPhone 3GS, iBoot-359.3 is considered the old bootrom. iBoot-359.3.2 is considered the new bootrom.