That could mean any number of things. Do they apprear only in games? Or is it an all the time thing? If it's all the time what happens if you open a game?
A few things you can try:
- A fresh install of your video card drivers.
- Check the temps of the GPU.
- Check the PC with another monitor or TV, see if those lines are still there.
- Try a different cable.
- If you have another PSU you can quicky plut in (don't even have to remove the new one from the case, just swap the cables for a few minutes).