Line across monitor

I have a 22 inch Hans-G HDMI monitor, and an hp slimline s3321p with vista home premium. recently there has been a black line that has just appeared on the screen. i thought maybe it was a virus or something but symantic didn't find anything and it doesn't do anything else, my computer is fine except for the line. i also thought it could be the monitor, but it goes away after about fifteen minutes if I'm doing something. i also read a forum that said it happened on a bunch of laptops and i could be because of the integrated graphics card. Any advice would be nice or how to fix it, thanks.