My computer is about 4 months old now and runs windows 8. I have downloaded OniGiri and have launched the game and, when I went to login, it says "downloading updates." And has been like this ever since. It's only gone up to 4% in the past 3 hours I have been running it.
Advice?

It will depend on your internet speed. You can try closing it and starting again to see if it goes past 4%.

I'm sorry to hear that you have encountered an error while installing Onigiri.
I think it has something to do with your internet connection speed.
If that's not the case then your Installer might have been damaged or broken, or it may be caused by a network connection while you were downloading it.
Please try re-downloading the installer, and make sure that you are not running any anti-virus or other security software that might interfere with the game's ability to connect to the internet.