It's just a hunch but you could be using something other than the latest java runtime for 64-bit machines.
I was having problems until I installed the 64-bit version and set the PATH to point to it.

Try this:

java -showversion -jar start.jar

It will run the same and you'll get the same error but it will show the java version.
It should look something like this:

java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)