White band in middle of screen

To try and narrow the source of the problem here are a few things you can try.

Do you get the white band in the BIOS setup screens?
If you do then it is a hardware problem.

Also try connecting an external monitor. If you still get the band on an external monitor then it is probably some kind of software problem.

Try booting from a linux live cd. There are heaps of different versions you can download.
If you can boot linux from a cd no problem then it isn't hardware

If it is software you can decide if your what to try and fix it or nuke it and install from scratch.
If it is hardware you might have to get a quote to have it repaired and then decide if it is worth it.

This is of course assuming you are out of warranty.