Well, I've never really had this problem before, but it's recently occurred when I reformatted my computer. Every single JAR I run receives that error. I've tried setting the main class manually, loading MANIFEST files from my workspace, etc, nothing works. I don't want to use command lines, just as a personal preference, or I would. I'm using Windows 7 64-bit, too, if that's any help. I've looked around and noticed people saying to change the directory program to JavaW, but I'm not sure how to do this. If you think that is a good solution, could you please explain in depth how to do so?
You probably need to set the file association for Jar files. Here's an older post that has some info on how to do that with older versions of Windows. I don't know for sure if Windows 7 changed this or not, but Vista used essetially the same thing.