In case some of you face the same error, when installing packages,

Warning: unable to access index for repository http://www.stats.ox.ac.uk/pub/RWin/src/contrib:
  cannot open URL 'http://www.stats.ox.ac.uk/pub/RWin/src/contrib/PACKAGES'

I deselected the CRANextra temporarily. May be there is something wrong with my network, but I also checked from a free proxy.

> setRepositories()
--- Please select repositories for use in this session ---
 1: + CRAN
 2:   BioC software
 3:   BioC annotation
 4:   BioC experiment
 5:   BioC extra
 6:   CRAN (extras)
 7:   Omegahat
 8:   R-Forge
 9:   rforge.net
10: + CRANextra
Enter one or more numbers separated by spaces, or an empty line to cancel
1: 1 2

As noted in the FAQ:

Some CRAN packages that do not build out of the box on Windows, require additional software, or are shipping third party libraries for Windows cannot be made available on CRAN in form of a Windows binary packages. Nevertheless, some of these packages are available at the “CRAN extras” repository at https://www.stats.ox.ac.uk/pub/RWin/ kindly provided by Brian D. Ripley. Note that this repository is a default repository for recent versions of R for Windows.

> sessionInfo()
R version 3.3.2 (2016-10-31)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

locale:
[1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United Kingdom.1252   
[3] LC_MONETARY=English_United Kingdom.1252 LC_NUMERIC=C                           
[5] LC_TIME=English_United Kingdom.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] tools_3.3.2