Java is not set up properly on X64 operating systems and needs to be added to a command line in order to launch the client prompt.
To start with locate your Java directory. It should look pretty much EXACTLY like this: C:\Program Files (x86)\Java\jre6\bin
Then you have to access your enviromental variables you do this by right-clicking "My Computer" and click properties then on the left hand side it should say "Advanced System Settings", head there.
Last but not least click on the Advanced tab and then click Enviromental Variables, from there search for the Path variable as shown below.
Simply add the Java directory to the end however seperate the previous and new variables with a SEMI-COLON. Hit ok, ok, ok etc etc and try and re-launch the desired Java client. If it dosen't work try restarting your PC (I had to whereas others didn't) and various java related bugs and programs should work.
The large pictures are there for those who have a little more trouble grasping simple tasks ;D












