Removing the yellowing from plstic is one of the most debated things among collectors. There is no good answer, sometimes if the yellowing is not too deep you can remove it with a scrubbing pad and some sort of abrasive powder or gently using sand paper. Try to work in circles as you want to minimise the scratch marks.
There is a far more dangerous solution, WD40, but it could turn your computer into a Dali art object so I take no responsibility for suggesting it...
A good solution is spray paint. It creates an interesting and unique colour to your already unique computer. BUT, this probably has a negative effect on it's value as a collectable...
