And if that doesn't work, sometimes auto detection doesn't, you can check it manually. Download usbview, plug in your iphone, put it in dfu mode, and you should see your apple device towards the bottom in usbview. Click it and then on the right should tell you some numbers at the end on one line. If it's iboot 359.3.2 or higher it's a new bootrom. If it's iboot 359.3 it's an old bootrom.
Not too many 3gs iphones has old bootrom. You can also do it this way :
Put iphone into dfu mode
Click start > type device manager
Universal Bus Controller
Select apple connected device > properties
Dropdown menu > Select Device Instance path > bottom cursor moves to right > iboot will be after the ECID