Back in the bad old days computer monitors used to be prone to a phenomenon known as "burn-in". Newer CRT monitors seem less susceptible to burn-in, such that I have not seen a case in five years or more.
Until recently that is.
Over that last year or two I have replaced all of the monitors in the house with LCDs. Imagine my surprise to discover that LCDs appear to be showing burn-in; some after only a few months!
Anyway, a quick google reveals a method of removing burn-in (Actually, known as Persistence on LCDs), which involves displaying an all-white image for a while. Twenty minutes works for me.
So, if you have this problem, click on the image below to view the full sized version (1280x1024). Save it to your computer. At this point you can either set it as your wallpaper for a while and minimize all windows (Desktop icons would remain though), or use it as a screensaver.
But read the article linked above - especially about lamp lifetime and blanking the screen. I don't recommend using the image as a background or screensaver all the time. Just when required.