if you have a 3GS you need to know what bootrom you have to see if you can jailbreak it or not. This does not apply to the 3G or the 2G because they don't utilize this system in preventing you from flashing a n older or custom firmware.
1) Put your iPhone i DFU mode; learn how to do that
here. Remember, if you see the itunes logo and USB you're in recovery. Do it again to get into DFU (blank screen, and computer recognizing it in DFU)
2) On your computer go to
Control Panel > System > Hardware > Device Manager
3) Find
"Universal Serial Bus (USB) Controller" and hit the
+ next to it to get the sub-menus
4) Go to
Details and then
Device Instance Path, scroll all the way to the right and you should see your iBoot version.
If it says 359.3 then you have the old bootrom
If it is anything above that, you have the new bootrom
