Hi there,
try and restart using the original System CD...do the lines still appear ?
If you have a local Apple Store that may be easier...as they can 'see' it.
The first thing I would do is check that it's not a software problem...(as above)
If the lines still appear after booting from the original disk, then the chances are it's a hardware problem...which is hard to diagnose from here. It mayjust be a lose cable, or the screen may be on it's way out.
regards
Ric