Hyperlinks to JPG opening in IE

I have hyperlinks in an Excel worksheet to various JPG photos. A
typical link looks like this:

file://C:/Docs/....etc...Pictures/Example.jpg

or sometimes like this

...\..\SUNDRY\CD+DVD\xyz.jpg

When I click one of these it normally opens in IrfanView, which is
associated with JPG files (I've checked the File Types list and that's
still OK). But today it has started opening in IE7! That's not even my
default browser (which is Firefox).

Anyone have any idea what's happening here please?

--
Terry, East Grinstead, UK