Very strange iBook G4 problem

The iBook is a 1.2Ghz G4 with 512MB RAM and 30GB HD and Airport extreme.

When i press power it chimes and starts booting but the screen is black and no apple logo.

After a small bit it gets the the startup screen of OSX and asks for password.

I enter password and then i get a weired screen like the bottom is black, above it dark blue and then a light blue on top and the mouse pointer but nothing else. It looks like a corupted screen as it is like patches, strange to explain.

I've tried resetting PMU and PRAM and the CMD SHIFT A V but didnt help.

I've booted of a tiger install disc and everything looks fine except that when it was booting just a black screen and no apple logo.