how can I clear screen burn in?

my 2.4gHz 24" iMac has been acting odd recently.

First a Christmas Eve HDD Failure, difficulty mounting the external hdd with the time machine backups on it and now a bout of screen freezing on shut down.

I suspect it is nearing the end of it's service life and am now lamenting my choice of buying a factory refurbished iMac over a new one that potentially could have suffered the same problems as this one as it reached the same age.

I probably would have been more accepting of the screen freezing issue had it not been for the residual screen burn in it has suffered.

Does anyone know if a recent burned in image will dissapate over time and use or if I am able to correct it somehow like with degaussing ?