When you association files with an application it's proper for this to happen.
Here's the old JPG association from Doug Knox. Should be fine for XP, Vista and 7.
http://www.dougknox.com/xp/file_assoc.htm (use the one for JPG)
I dont really know why I installed this but I did.
The damn program has converted all my jpg files to Serif Photoplus pictures.
I am sure I did not authorise that.
How can I convert everything back to jpg? There are hundreds of files.
And Serif now seems to be the default for all pictures.