As Lilybiri says, PowerPoint uses inches or cm instead of pixels. Why they do this is a mystery lost to the ages. Fortunately they use 96 pixels per inch, which is why the default 10in by 7.5in is 960x720 when imported to Captivate. You can do some simple math and resize the file before you import it, by going to the "Design" tab and selecting "Page Setup" on the far left. (1024x786 is 10.667in by 8in)
Personally I don't much trust PowerPoint or Captivate to rescale without some funky formatting issues (fool me once...), so I export the PowerPoint slides as images (File>Save As, then select your preferred image type from the drop-down menu) and use Photoshop to batch resize the images. You lose the animations and videos this way, of course, so I don't know that it'd work best for your situation.
Hope that helps!