I?m currently on a diet and exercise program, and I take a photo of
myself from the front and side every Sunday.

I save put the photos into the same folder, and use the ACDSee file
viewer to make a slideshow. However, because I don?t line myself up in
the exact same position each time, the slideshow has me moving
slightly left or right each time the image changes.

I?d like the images to line up perfectly, just like you see on those
infomercials for exercise machines.

How can I get the images to line up perfectly?

You have to use a morphing application. But it won't be automatic anymore!
You have to select the picture to animate, then select some common
point (such as the corner of your eyes, mouth, your ears...) and then
you can animate the to pics... The first image will be transformed to
become the second one.
I'm sure there is some Morphing apps which allow you to do it with
more than two pics.
Is that what you want to do or not??

Please forgive my english which is quite bad....

What u can do is, keep on using ACDSee... just draw a mark where you
will stand and mark the point where u put you camera which took your
photographs. In this way even if you keep on using the same software
you will not feel that much of the physical displacement of the body.

Another thing that u can do is, make a powerpoint slide show, where
each silde contains two images, one from the side and one from the
front...
just keep on adding the images everyday with a date associated to
each. You can also create two differenc slide shows one with side
images and the other one with front.

In this way you would not be required to maintain the folders.

There are a number of ways to do this. The object is to take your
photographs, that are each slightly off from the last, and crop them
so that your figure is in the exact same place in each frame. here is
how I would do it.

1. Open the first image in Photoshop.
2. Open the next image then drag the Layer from the second image to
the first image. This will create a new layer in the first image that
contains your second image.
3. Change the opacity of the second layer so that you can see through
to the first image layer. using the Selection tool, click and drag the
second image (now a bit transparent) so that the eyes nose and mouth
align perfectly.
4. Return the opacity on the second layer to 100%.
5. Continue to add your new images to the first, creating a new layer
for each. Align each new image layer perfectly.
6. Once you have the layer images aligned, turn off all layers but the
first using the Eyeball icon at the left of each layer.
7. With Layer 1 turned on, Save For Web and save your first image layer as a JPEG.
8. Continue to turn on and off the layers to save each layer image as JPEGs. 
9. Now place these images in PowerPoint. They are each exactly the
same size and perfectly ligned.
10. In PowerPoint, double-click each image to make the Format Picture
menu open. Check that each image is 100$ and that at the Position tab,
the Horizontal and Vertical measures are the same for each image on
the slides. That way the images won't jump around as you play them.
11. If you want the slides to "morph" into each other, select a slide
transition such as cross fade.