Hmmm... most likely an environment issue. A couple of questions:
1) is the Java bin directory in your path? If so, good, if not you'll need to add it to your profile. You'll know it in your path if you can simply open a telnet window and type java without a path, or better yet, type 'which java'. Which will run down your path and tell you the location of where it found the java binary. You might find you have java in your path, just not the correct one.
2) Do you have JAVA_HOME environment variable set? You may also need to set JDK_HOME. I have it set (both to the same directory) but not sure if it's really required for Eclipse.
My best guess is that your problem is one of these. Of course, I'll be the first to admit that I don't know everything about everything, so I could be way off base