If the hard drive can't be regonized or too old to use, then the what happens when the cpu usage is at 100 error may pop-ups. Make sure everything about hard drive are ok then you can pass this possiblity, well, a little more, regular maintenance and good using habit will do good to increase its lifetime.Third -- Unreasonable startup or shutdown operation.
Have you ever forced a shutdown operation to your computer with programs running? It's quite bad thing to do, violent and high-risk to computer, and yes, it will bring about what happens when the cpu usage is at 100 error.Fourth - Baddly programmed third party applications.
Baddly programmed application is likely to be a reason of what happens when the cpu usage is at 100 error, when users install or uninstall programes, especially uninstall some applications, some vital or important data/files may be deleted accidently. Next time, when you launch the personal computer or open a program, that file can't be found, then error will pop ups.