Whenever you use a tool to capture images from your screen (Alt+Printscreen, screen capture from EverNote, etc.) and then pasted the captured image onto the canvas, it causes the right 3 columns of pixels to be removed from the right of the image and become the leftmost 3 columns of pixels in the image.

This is using Windows XP 32 bit.

I attached an image of the a screen capture I took of a blue and green rectangle next to eachother to try to help explain a little bit better what is happening - please note, this was only one blue and one green rectangle before I used a screen capture tool and pasted it onto a new canvas.

Feel free to follow up with me if this doesn't make enough sense.