I'm on Windows 10, and I've two libraries:
C:/Program Files/R/R-3.5.3/library
C:/Users/Username/R/win-library/3.5
The first one contains the libraries that came with the R 3.5.3, and the second one contains several other packages that I downloaded later.
I don't have that any library in the corresponding location of Z:\R\R-3.5.1\library. What packages do you find here?
If there's no packages, I think you may remove this folder.