Make, model, type, etc. LCD or CRT?..and by scanlines do you mean some sort of vertical or diagonal thin traces that are there constantly or are they changing? My first thought is to try the monitor on another machine if possible and/or check your PC with another monitor. This considerably narrows the possibilities. You rule in or out what's the easiest to do. You can also see if the anomaly exists while running in safe mode. If you still have the scan lines in safe mode, it's more likely the monitor or display card. If not, you start looking at the driver, some setting, or the monitor's failure to respond to it properly.