Hi Dingaling
There's no need to uninstall the existing software. (However, if you're sure that you're not going to use it, you might as well do so).
When you install something like Irfanview, it will probably ask if you want it to be the default program for viewing images. I suggest that you agree to this. (You can easily change it to something else later if you want to do so). Then, when you double-click on an image file, it will automatically open in IrfanView.
IrfanView is great for simply viewing images and is quite good for editing them as well. However, you might want to consider adding a dedicated image manipulation program. I use Irfanview for viewing images (and some editing processes, such as removing 'red-eye') but I use Photoshop for most image editing. I'm not suggesting that you should get Photoshop (because it's expensive) but you might want to consider a free product, such as PhotoPlus 6.0, to work alongside Irfanview:
http://www.freeserifsoftware.com/
Whichever software you use, your card reader should make it much easier to keep track of where your images are. You can now simply drag your pictures into one common folder or, if you so choose, separate folders for 'Family', Holidays' 'Pets', etc.
Chris