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

[Jon Olav Vik]

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 with the message

The instruction at "0x7c9106c3" referenced memory at "0x7c9f4302". The memory 
could not be "written".

The same problem occurs when calling help from the command line, e.g. with
 ?license

I'm running R 2.6.2 on Windows XP SP2. Googling for the error turns up nothing, 
and I have no idea where to look for further diagnosis. I have uninstalled, 
deleted the R package directories, and checked the registry for metions of 
"\R\", but the problem remains. I can't think of any unusual changes I made 
before this problem arose.

Any help would be appreciated.

Best regards,
Jon Olav Vik