If they are always in the same locations then your monitor is failing and needs to be replaced. If they move around it might be your video card failing. Either way, you should connect the monitor to a different PC to confirm it is the monitor.
If it is a screen with Passive Matrix technology, it probably means there are burnt transistors (dead pixels)....they show up as whole lines. In a screen with Active Matrix, only one dead pixel shows up (and not a whole line).