Thanks to the Slide Show toolbar, new in PowerPoint 2003, moving forward and backward with the mouse is easier than it used to be. The toolbar, shown in the picture on the left, includes forward and backward arrows. While it's always been true that you just click to advance to the next slide, going backward hasn't been as simple.
This toolbar appears only if you move the mouse over a slide. It's displayed for a few seconds in the lower-left corner. In addition to the forward and backward arrows, it offers pointer, navigation, and display options on a couple of menus.
So that the toolbar doesn't overlie your slides, it fades away when you're not using it, and it comes back gradually so that it's not a distraction. You may have to wait a second after moving your mouse before you can click the toolbar.