Try pointing it to the new msi file. I had to do this once before without the old msi. I think the installer played up though, not installing the latest version and leaving your machine without PGP Desktop - DON'T restart if this happens because Windows won't be able to boot. Instead, run the installer again and it should go on ok.
It's been a little while since I had that problem, so I'm going off memory, but give it a try.