Of course you can replace your CPU. Since we got the example with the movie, check this, does your CPU remain at 50-80% during the movie or only at the movie's start? (i mean when you open the file)
Again it sounds to me like a hard disk issue and not CPU's. And even if it's running like 50-80% you shouldn't freeze, maybe lag a little bit but not freezing. And that sound is most likely caused by your hard disk.
Why don't you try removing and putting your disk back, or even better replacing the disk with another one. I'm not saying to buy a new one since we are not sure about your problem, maybe you have another one, or you could take your friend's?