You can edit Rprofile
in the base
library (in 'C:/Program Files/R.Files/library/base/R'
by default) to include code to be run on startup. Append
######## User code ########
.libPaths('C:/my/dir')
to Rprofile
using any text editor (like Notepad) to cause R to add 'C:/my/dir'
to the list of libraries it knows about.
(Notepad can't save to Program Files, so save your edited Rprofile somewhere else and then copy it in using Windows Explorer.)