Basically, what I did was install the JRE on a USB stick, using the offline installer.
Now, the Java installation on the computer I used to do this refuses to be uninstalled or updated -- because it can't find the installation files. Understandable, since the USB stick is not plugged in, and never will be (it was stolen since).
So, in essence, Java firmly believes it's already installed, but the files are gone. How do I proceed?