Error: file association for 'doc\html\index.html' not available or invalid

 > Today HTML help stopped working. The menu command "Help > Html help" usually
 > brings up my default web browser (Opera 9.26), but now R gives the error
 > Error: file association for 'doc\html\index.html' not available or invalid
 > If I try the same menu command a second time, R crashes

[...]

 Try re-setting the file association for .html files -- it looks as if it
 has been corrupted (not by R).

Blimey. It worked.

I'm sorry I didn't mention it in my first post, but *.html files did
open with Opera even while R would crash. However, following your
suggestion, I looked at the Properties for an html file, and it said
"Opens with: unknown application". (Strangely enough, the file icon
was still Opera's "O".) So I re-selected Opera as the default
application, and everything went back to normal. Thank you so much for
your help!

Jon Olav