change default pdf viewer

Hi everyone,

I try to change the default pdf viewer that is used by ess (emacs speaks statistics).
When I compile LaTeX file via the command M-x P choosing pdflatex, the acrobat reader shows up.
I tried changing to SumatraPDF via M-x customize-group RET ess RET and changing the value of 
ess-pdf-viewer-pref, but still acrobat show up. This is been saved to my iinit.el (after 
customizing and saving for future sessions):

  '(ess-pdf-viewer-pref (quote ("C:/Programme/SumatraPDF/SumatraPDF.exe" "-reuse-instance")))

Hope anyone can help.

Best
Oliver