I don't get that error.

I would normally expect the error to give a file name, since it says "the system cannot find the file specified." But it just says 'C:' which is a path with no file name. I suspect you have something in your library with a bad path so the filename is missing. If it's only when you open your materials library, and especially in a particular runtime, that might narrow down which file it is. If the file is actually missing, permissions won't help.

Are there any items there with a missing thumbnail? That's not the only thing it could be, but it's one possibility.

If it is a permissions issue, you should be able to get around it by running Poser as an administrator. Right-click on the Poser icon (you might need to hold down the shift key) and pick "Run as administrator." If that doesn't prevent the error, it's not a permissions issue.