Opening R in 64-bit version by default

 .... but really, it's just a bug.

If you manually change that registry key, things are fine.  If you  
ask Windows dialogs to do it for you, it fails.

Most people pay infinitely more to Microsoft for Windows than they  
pay to R Core for R.  I hope that's also the ratio of their  
complaints to Microsoft about this bug to their complaints to us  
about R.