I can't launch Onigiri it keeps saying that it has stopped working. I tried to download again and tried GothicKitty's method but it does not work too. It only happened after I downloaded Windows 8.1, maybe it has something to do with it?

Before anyone says something something windows 8.1, I'm running the game just fine. Try running as admin? No clue.

Personally use a Windows 8.1 myself, it has nothing to do with it, you can trouble-shoot the client and see if it fixes.
If you're still having issues, attempt to remove(Using control-panel) and re-download again from the main-site, prefer if you restart the computer first.
These aren't real solutions, since unsure what caused the issues for you. If you play the Onigiri_US, hopefully it didn't download files from the JP-Onigiri server, there is some cases which myself has experienced where the client downloaded from the JP-server which has "destroyed" the game, we were no-longer able to open the client to run the game, had to delete and re-download everything.
To better understand what happened to you, would ask for a screen-shot.
~Thank you