Go into your control panel (Start Menu- control Panel) and select the option for setting the default programs. And change the default program for opening all of the files you don't want Instant Viewer to open. It's a bit annoying but that's what I did and I don't regret it in the least.
Other than that, I don't know of any way to uninstall it. Just change what files it can open.
If you click on the help button you'll see that Corel's own help files suggest something similar. Just a little different method for doing the same thing.
Changing the Default Photo-Viewing Program
When you install Corel PaintShop Photo Pro, Corel Instant Viewer becomes the default program for viewing photos. You can change the default program at any time. For example you can set Corel PaintShop Photo Pro as your default viewer.
To change the default photo-viewing program
1 Right-click any photo on your computer.
2 From the context menu, choose Open With > Choose default program.
3 Choose one of the recommended programs, or click the Browse button to find other programs.
4 Enable the Always use the selected program to open this kind of file check box.
5 Click OK.
Next time you double-click a photo in Windows Explorer, the photo will open in the default program.