Well, ever had a slow computer, then you should run defragment programs on it.
But to they work?

In my experience diskeeper, have seriours problems with stability.
I have tried to install it on severel computers, with the same result.
Slower upstart, and a boot optimizer that did work, sice it just stalled, and did nothing fore severel hours.
I can only warn about this program, but potential users.
The tests have been done XP pro vista 64bit.