Is it possible to add an adjustment layer just to one layer in Photoshop? I've tried, but the adjustment layer seems to affect all layers.
Yes it is possible in CS4 and (I assume CS5 though I don't have a copy). By default the layer affects all the layers beneath it, but if you click on the adjustment and bring up the adjustment window (where you set the properties of the adjustment) there's an icon which says
"This adjustment affects all layers below (click to clip to layer)"
Clicking on that icon sets the adjustment layer to apply only to the layer beneath it. See the following quick screenshot.