Let's rule some basic things out, Do you have R already installed in your system? if so, what version? and how have you installed it? have you modified the default installation settings in any way? Do you have any custom startup files (Rprofile.site, .Rprofile, Renviron.site or .Renviron)?
Have you tried resetting RStudio's state?
Have you tried the latest daily build for "Elsbeth Geranium "?