When I try to run Minecraft on my Windows 8 computer, it comes up with a error that says: The registry refers to a nonexistent Java Runtime Environment installation or the runtime is corrupted. The system cannot find the file specified.
I fixed this for myself, after days of tribulations:
Adding the Java thing to the Path variable didn't make a difference to start, and typing in 'java -version' in cmd posted an error. However, that error showed that it was looking in the E:\ drive for the AMD64 Java, which wasn't installed. Additionally, trying to install the AMD64 Java resulted in a silly 'invalid character' error for 'Program Files', so it couldn't install.
I fixed it now though, by regediting Computer/HKEY_LOCAL_MACHINE/SOFTWARE/JavaSoft/ and then each time that the E:... directory showed up, I replaced it with C:\Program Files (x86)\Java\jre6. Then, the AMD64 Java install worked, wonderfully, and everything else then worked, including 'java -version' and Minecraft.exe.