If this is something that slowly got worse then chances are the CRT's are coated with dust/dirt/grime. You could spend a few hundred and get an local repair shop to clean them and the mirror, or call an ISF pro out to clean and calibrate for twice that or buy a new set. A smaller rear projection set like yours isn't worth probably what it would cost to repair. But you aren't selling, just viewing, so it's a judgement call on what to invest.
If this problem suddenly popped up, then it could end up being something entirely different and more expensive. Good luck.
http://www.keohi.com/keohihdtv/learnabout/generaltips/screenoptics/cleaningtheoptics.html