You need to provide some information - like what OS are you using, where was Java installed...
If you are using Windows, check the installation in Program Files, typically it would look like C:\Program Files\Java\jre1.5.0_xx. Inside the bin folder, there is an executable called javacpl.exe which, on double-clicking, will open the java control panel. Try changing the options.
Another suggestion I could think of is to try and recover your Windows registry.
Happiness is good health and a bad memory.