After a recent hardware overhaul, Reader has refused to update.
It was originally installed onto the C:\ drive.
The same drive still exists untouched.
Reader and everything else still works fine.
The original install app was on J:\ which now doesn't exist
(the drive, the letter OR the app)
and I get Error 1327 Invalid drive J:\.
Furthermore, I can't un-install it because of the same error.
I've solved it.
I shoved in an empty flash drive, re-mapped it to J:/ and it got past the error.
Thanks to anyone who would have suggested that, if you gat a chance.