For some reason, people treat a program consuming 100% CPU
as if it were unrepentantly running around kicking defenseless
(and cute) puppies.
Calm down already.
I get the impression that people view the CPU usage column
in Task Manager not as a diagnostic tool but as a way of
counting how many puppies a program kicks per second.

While a program that consumes 100% CPU continuously
(even when putatively idle) might legitimately be viewed as
an unrepentant puppy-kicker,
a program that consumes 100% CPU in pursuit of actually
accomplishing something is hardly scorn-worthy;
indeed it should be commended for efficiency!

Think of it this way: Imagine if your CPU usage never exceed 50%.
You just overpaid for your computer; you're only using half of it.
A task which could have been done in five minutes now takes ten.
Your media player drops some frames out of your DVD playback,
but that's okay, because your precious CPU meter never went all the
way to the top.
(Notice that the CPU meter does not turn red when CPU usage exceeds 80%.
There is no "danger zone" here.)

Consider this comment where somebody described that they want their
program to

use less CPU but get the job done reasonably quickly.
Why do you want it to use less CPU?
The statement makes the implicit assumption that
using less CPU is more important than
getting work done as fast as possible.

You have a crowd of people at the bank and only ten tellers.
If you let all the people into the lobby at once, well, then all
the tellers will be busy—you will have 100% teller utilization.
These people seem to think it would be better to keep all the customers
waiting outside the bank
and only let them into the lobby five at a time in order to keep
teller utilization at 50%.

If it were done when 'tis done, then

'twere well / It were done quickly.

Rip off the band-aid.

Piss or get off the pot.

Just do it.

If you're going to go to the trouble of taking the CPU out of a
low-power state, you may as well make full use of it.
Otherwise, you're the person who buys a bottle of water,
drinks half of it, then throws away the other half
"because I'm thinking of the environment and reducing my water consumption."
You're making the battery drain for double the usual length of time,
halving the laptop's run time because you're trying to "conserve CPU."

If the task you are interested in is a low priority one, then set
your thread priority to below-normal so it only consumes CPU time
when there are no foreground tasks demanding CPU.

If you want your task to complete even when there are other foreground
tasks active, then leave your task's priority at the normal level.
Yes, this means that it will compete with other foreground tasks
for CPU, but you just said that's what you want.
If you want it to compete "but not too hard", you can sprinkle
some Sleep(0) calls into your code to release your
time slice before it naturally expires.
If there are other foreground tasks, then you will let them run;
if there aren't, then the Sleep will return immediately
and

your task will continue to run at full speed.

And cheerfully watch that CPU usage go all the way to 100% while
your task is running.
Just make sure it drops back to zero when your task is complete.
You don't want to be a task which consumes 100% CPU even when there's
nothing going on.
That'd just be kicking puppies.

[Raymond is currently away; this message was pre-recorded.]

Clarification:
Many people appear to be missing the point.
So let's put it more simply:
Suppose you have an algorithm that takes 5 CPU-seconds to complete.
Should you use 100% CPU for 5 seconds or 50% CPU for 10 seconds?
(Obviously, if you can refine your algorithm so it requires only
2 CPU-seconds, that's even better, but that's unrelated to the issue here.)

A project many years ago neared the conclusion of one of its
project milestones.
Things were getting down to the wire,
and upper management was concerned that the project may not
reach the milestone on schedule.

To ensure success, they decided to send email.

From: Senior Manager
Subject: READ NOW!!!! More than one bug

Please see the attached spreadsheet.
If you are on the To: line you can look at the Assigned To:
column and find your name.

You are in this spreadsheet if you have 2 or more bugs assigned
to you.
At this stage of the project as we are winding down and entering
Milestone Z,
people with a high number of bugs assigned to them end up being
the

long poles
for the release.

We have a lot of teams getting to zero bugs, so we have people
who can help you.
If you need help, then ask your manager.
Your manager should be on the Cc: line.
It's

key
we get all the bugs fixed (with quality)
as soon as we can.
If your manager agrees you can use help,
and doesn't know where to go for help,
then ask me.

Because as we all know, you can get people to work harder
by sending email.

I happened to be one of the recipients of this message,
and I sent back a simple reply:

I have more than one bug because my manager asked me
to help out other people who have more than two.
Thanks for your concern.