Hello David, I'm Johrelle a fellow Windows/Microsoft user. I'm happy to assist you.
Ensure that the C: drive or specific folders you want to access are shared and that your user account has the appropriate permissions to read/write on that share.
Right-click on the C: drive or the specific folder, go to Properties -> Sharing -> Advanced Sharing, and make sure the folder is shared and your account has access.
You can also try running File Explorer as an administrator and then try to access the share. UAC might be preventing access even though you're an administrator.