It's hard to even try to help without proper info.And also, I'll have to put on a disclaimer/signature "I'm new here too :D"
What error does flashtool give you ?Exact one.
In the meantime, google md5 checker, or download this one http://www...
I've installed the drivers from flashtool and still get this error
Sent from my D6603 using Tapatalk 2
Ok, don't worry. I see! Install again the drivers from here: http://singledrivers.blogspot.it/2014/09/free-sony-xperia-z3-dual-usb-driver-for...