Hi,
I agree with Ute and John. Look for the ample free training first.
But if you prefer to be guided through the learning process I can recommend
the tutorials from Lynda.com.
www.lynda.com
I didn't try the Powerpoint versions, but their Photoshop and Premiere
tutorials are marvellous. They work especially well if you have a dual
monitor setup because you can run the course on one monitor and follow along
with the application on the other.
I think you can download free samples to see if they suit your way of
learning.
Remember also that there are two independent aspects to learning an
application like Powerpoint. One aspect is the "mechanics" of knowing how
to achieve a particular task (like animating an object). The other is the
way you present the material and ideas. Good presentation (as opposed to
Powerpoint) practice is the same whether you're using video, slides,
flipcharts or a whiteboard. The presentation should be used to support the
ideas being discussed, not to replace them. There's a zeitgeist generally
referred to as "death by Powerpoint" that encapsulates these dangers...
http://en.wikipedia.org/wiki/Death_By_Powerpoint#Criticism
Edward Tufte's "Cognitive Style of Powerpoint" is probably the most famous
example of a book devoted to the problem.
So don't be tempted to skip on the tutorials that offer guidance on
presentation style, rather than mechanics, they're important
Cheers,
Geoff