MARKALLAN  Which virus was it? Do you still have the file? Don't just say it sucks, do something about it - submit the file to http://cgi.clamav.net/sendvirus.cgi and let them know that another AV program picked this up whereas ClamAV (the backend behind ClamXav, remember ClamXav is only a user interface to other software) didn't.  
(Version 1.0.4)

MARKALLAN  Check your international settings in System Preferences -> International -> Language and make sure English is at the top (if English is your main language) or put the list in the order you prefer, then try launching ClamXav again.  
(Version 1.0.4)

MARKALLAN  So, after you've copied it off the disk image, it simply refuses to run? Do you see the main window at all or does it just not appear and nothing happens?

Where are you copying it to? Do you get anything helpful if you open the Terminal and type /ClamXav.app/Contents/MacOS/ClamXav  
(Version 1.0.3h)

MARKALLAN  As well as the piecemeal engine updater, did you also download the ClamXav application itself? It was listed on that same page as "Application Only".

The main news here is that the app was updated, the fact that the engine was also updated at the same time is purely coincidental!  
(Version 1.0.3h)