I think I've noticed Cindy describe the steps for manually editing the registry to resolve this problem over in the beginner or intermediate forum. I'd suggest trying a quick search for that
thread.
Otherwise, you might have some luck uninstalling all JREs you've installed, restarting the system, and then installing the newest JRE only. I've had success doing that, but I've actually seen that strategy fail once, too.
As a generally good idea, you'll also want to make sure that Microsoft's JRE isn't interfering. You can usually do so by deleting or renaming any java.exe files in a Windows or Windows/System or Windows/System32 directory. (I don't think this relates to your current problem.)
Any luck finding Cindy's thread or with a reinstall?