If you want to protect yourself against losing data because of sudden hard disk crashes, give HDDlife a try. It constantly monitors the health of your hard disk, and warns you when it uncovers problems--something as simple as your hard disk running a bit hot. With that warning, you'll know that you should back up your hard disk, and take some kind of action to protect your hard disk.
The program runs in your system tray, so you can see its health at a glance. It also includes a gadget that runs in the Windows Vista sidebar. And it includes extras as well, such as reporting on the speed of disk reads and writes, and even lets you quiet your hard disk if it's running too loudly.
Note: After 15 days, the software reverts to the free nagware version, which doesn't include some advanced features, such as showing the temperature of your hard disk, and showing the health status of your hard disk in the system tray. To keep those features, you'll have to pay a $35 registration fee.