Download the program, And when your installing it, You have the option of associating the program with your image files. Enable it.
Then open the image using irfanview, And press the ' - ' key a couple of times to reduce the size. When you are happy with your result, Go to:
Image > Resize/Resample
Put the corresponding dimensions in the two boxes (Width and height), Which you will find in the top left (It automaticly adjusts itself as you press the ' - ' key) Click ok, Then save as you normally would, rewriting over the old file.