It's probably the monitor, since that sounds a lot like pretty classic LCD failure. Usually it's a horizontal stripe that occurs, but if it's an older monitor, they might have laid the LCD strips out vertically.
my lcd screen is fine upon start up and stays good for about 10 minutes, and then multi-colored vertical stripes form over about 20 seconds and remain. I restart, but the problem doesn't go away until the computer sits for several hours.
It's a P4 running XP and the LCD is about 6 years old, off-brand "SYS"
Is this something to do with heat build up that's affecting one of the components?