Step 1
Create a new document and paste any image onto the document. You can use the one on this page if you like. Just right click on the image and click "Copy Image". Next goto photoshop and press Ctrl+V to load it onto the document.
Step 2
click on the Magic Wand tool and click somewhere around the object in your image. In this case, the white color around the PC.
Step 3
You should now see your selection plus some parts that shouldn't be selected. Since we only want to delete the white background we need to unselect the parts that are not apart of it. click on the Rectangular Marquee tool and Alt+Drag around all the spaces that you don't want selected.
Step 4
Now that your selection is completely around your object, press delete. The color around the layer should be gone and you should only be left with the pc. Now you can manipulate the image without having to worry about interfering colors around the object. I added a simple background.
Final Result:
