So I guess you mean the screen has a blue tint to it?
Did you install any sort of drivers and then it turned blue? I'm thinking it's so old you didn't do this, but doesn't hurt to ask. It's possible if you installed some drivers you're getting the blue tint. However, given the age of the machine, I'm inclined to think hardware.
Do you have another monitor you can test with it? Your monitor may be going bad. If switching monitors works, you need a new monitor.
Do you have another cable you can test with it? Your cable may be going bad. If this works, you need a new cable.
If none of the above, then it's the GPU (I think they had onboard?) that's gone bad. Live with the blue tint or buy a new computer.