Looking at the error message, is nexus mod manager installed in your appData folder? That is a protected folder so most likely it's a permissions thing. This is where mine is installed D:\Nexus Mod Manager. I would uninstall and reinstall depending on how many mods you are running it's fairly easy. I am using Winrar also and never have an issue. Here is a video tutorial on getting it set up and running. https://www.youtube.com/watch?v=puzGu4Cm2VQ