Put your iphone into DFU Mode as follows,
Connect the iPhone to your computer and launch iTunes
Turn the iPhone off (hold down the power button at the top of the iPhone)
Hold down the sleep/power button and home button together for exactly 10 seconds, then release the power button
Continue to hold down the Home button until a message appears in iTunes telling you an iPhone in recover mode has been detected.
Then you should be able to restore from a previous.
Then you must update to the latest firmware 4.3.3 unless you already havent done this?
Then use jailbreakme.com which is the best way to jailbreak the latest firmware.
Alternatively have you tried plusggin it into a different USB port:? Let me know if your having any more problemns after trying all of the above