I ran into the same problem recently when I needed to reinstall MP3TS on my new computer. I also emailed him and he did reply. Yes, it's a bit tricky to get it working on Win8.1 but I was able to do it.

The pop up message that appears if you click on the expiration screen and wait around... that popup screen that you complained that is not mousable... it's actually the actual About screen from the main program. Even though it is not mousable, you can perform the registration steps on THAT screen. You don't need to be inside the program to perform the steps to register it, you just do the steps on that popup screen. The steps are hold down CTRL and type R E G I S T E R if I recall correctly.

The parts that weren't clear to me, and what tripped me up were:

- After clicking away the expiration screen there is a 30-second pause before the About screen appears. I was task-killing the program when it didn't respond after about 15 seconds.

- When the About popup eventually DID appear sometimes, I had no idea that THAT was the same as the about screen inside the program where I was supposed to type CTRL-REGISTER. After he explained it to me in email I understood, but that wasn't made clear anywhere else.

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If you still have the old computer hard disk, you can just copy the file mp3ts.rg from the old installation directory into the new installation directory, and that works fine too. Skip all the stuff above.