[MiKTeX] mthelp doesn't work as usual

[Ignasi F. <ign...@ui...>]

Since some time ago (I don't now when) mthelp doesn't work as I was used.
When I wanted to look for some package documentation, just writing  
something like
'mthelp booktabs' in start->execute, the browser (Opera) showed a new tab  
with a link to dvi, pdf or any other possible document.

Now nothing happens and doing it from a command window next message is  
shown:

mthelp: The viewer could not be started.

Do you now how to solve the problem?

Thank you

Ignasi