Why do computer settings change on their own?

I would like to know why computer settings frequently change spontaneously for no apparent reason. At least once a month, whilst using Word 2007, I suddenly find my second row toolbar, the one with bold, underline etc., has gone and hidden itself again. One day last week my Outlook text was suddenly blue. The gadgets (clock, news and weather) on my desk-top ecided they no longer needed to be seen (of course I brought them up again). The point is why do these little random changes happen in the first place, often betwen shutting down and then restarting the next day (so of line)? I have had a couple of more serious ones as well. My default email program morphed from Outlook to Express a few months ago. I recall suddenly finding my line spacing had doubled and so on. Any comments welcome.