Weird text...please help!!!

Ok so I'm an IT intern at my company and I recently had to completely reformat a user's hard drive and set it up from scratch all over gain because of various viruses that this user contracted it was just easier to do so. Everything is working great except for one thing. We have a group policy that locks the users profile after 15 min of inactivity, after they come back they have to press ctl+alt+del and enter thier password again. however after the lockout and when the computer goes to sleep, the text on the screen (windows, icons, all text) looks broken and sort of like chicken scratch, the only thing that fixes the problem is a reboot. Is this a driver issue, or the video card? Any info would be greatly appreciated!!!