I have a problem that seems to be getting worse (It happened twice today) It seems to happen when I am scrolling down a screen. Today it was an email, then a webpage. All of a sudden, the screen freezes and the little turning colour-wheel replaces the mouse pointer. I have found that there is nothing I can do to get out of it. The only way, which I hate doing, is to pull out the power cord and then replace it and restart.
Does anyone have any possible explanation? Also, is there any key combination that can be used to quit instead of cutting the power?

What does the colour wheel signify as opposed to the tiny black and white wheel which I assume means that someting is loading.

Thanks