Member Rara Avis
While Grinch's list contains possible problems, the most likely culprit is a computer that has either a) gotten a virus (as he noted), b) gotten tired of having too much data swapped around and has developed a virtual form of gangrene, or c) a combination of a & b.
I would start with the simple first, then escalate after that proves wrong. I.e. I wouldn't worry about a bad motherboard, etc., until I ruled out the likely possiblity that the data on your hard drive is just jumbled; all those symptoms have happened to me at one time or another.
My response to any more than mildly irritating problem with any of my computers is to restore it. Most (if not all) computers that compe pre-loaded with software have restore discs. I usually try reloading it without a total format, but if that doesn't work, I just wipe the old and make way for the new. It has almost always solved my problems.