Has your default SL viewer been hijacked?
Well, you can fix that in regedit.
There are three or four entries you'll need to change, one per protocol.
Viewer name isnt really important, its mostly cosmetic.

>HOP:
VIEWER NAME: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\hop\shell\open - FriendlyAppName
APPLICATION LOCATION: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\hop\shell\open\command - Default

>Secondlife:
VIEWER NAME: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\secondlife\shell\open - FriendlyAppName
APPLICATION LOCATION: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\secondlife\shell\open\command - Default

>X-grid-info:
VIEWER NAME: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\x-grid-info\shell\open - FriendlyAppName
APPLICATION LOCATION: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\x-grid-info\shell\open\command - Default

>X-grid-location-info:
VIEWER NAME: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\x-grid-location-info\shell\open - FriendlyAppName
APPLICATION LOCATION: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\x-grid-location-info\shell\open\command - Default

Format of the 'default' link registry key is always:
"C:\Program Files\Singularity\SingularityViewer.exe" -url "%1"
%1 is always replaced with the url you are choosing to open