If the cooling system can't handle the CPU when at 100%, it ain't much of a cooling system. Maybe it needs the dust blown out, or maybe the fan has seen better days.
When I burn a movie on a DVD my CPU is at 100% for an hour at a time. No problem though. I can hear the fan, but it's not really loud. I design my computers to be quiet.
As has been mentioned earlier, the CPU seldom locks up. It gets stuck in a loop and run 100%. If the loop is in a system state program, like a driver, the result is the same though.
I rig my computers up so I can see the CPU activity at all times, independent of the OS or software. I find it interesting.
For instance, when you enter the BIOS, the CPU will run at 100%, waiting for a key press. I guess simpler is better in a BIOS.
Lot's of stupidly written programs will run the CPU at 100%. If I foolishly try to look at a .pdf file from my browser without downloading it, Adobe Reader will often run at 100% while it's downloading the file.
When you're installing a multi-disc Linux and it's waiting for you to insert the second disc, guess what? CPU 100%. Again, maybe simpler is better. Anyway it's harmless. The list is endless.
Running the CPU at 100% doesn't usually preclude you from running other programs. If all the system stuff is okay, the operating system will give each program it's time slice.
Another thing about frozen computers. Sometimes it's just the "console" that's frozen. That is the monitor is black and the mouse and keyboard have no effect. The OS can be running fine however. This has been happening to me lately when I totally log off from one account when there is another account active. I think it is a problem with the video drivers, but I'm not sure. As I have another computer on the LAN, I can log in from there and shutdown the "frozen" computer.