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)
I am getting
java version "1.7.0-ea"
Java(TM) SE Runtime Environment (build 1.7.0-ea-b143)
Java HotSpot(TM) 64-Bit Server VM (build 21.0-b13, mixed mode)
Hmmm. Well it was just a hunch. Sorry I couldn't be of more help.