Rhino 8 upgrade not loading/responding

Are there any errors shown?

Something is wrong with your Windows, is it fully up do date? Do have a aggressive virus protection software installed?

I would focus on the Rhino 6 install first, less variables.

is this a laptop with a hybrid graphics system?

here is a microsoft .net repair tool
https://www.microsoft.com/en-us/download/details.aspx?id=30135