I just got a new computer and now I have PSE 9! I used to use PSE2 and I must admit that I can't see much difference between the two, so I guess version 7 will be much the same as 9.
So, what I did to "mask off" in the paintover was to put in new brightness/contrast adjustment layer from the 'layers' drop down menu, then alter the brightness/contrast sliders to where I wanted them to be. Now in your layer stack there will be a new layer with a little icon on the left and a white thumbnail on the right. That white thumbnail represents the layer mask. At the moment it is all white, but if you paint onto that layer you will mask off the area under the area you paint. I think you can only paint in shades of grey to black, but the darker the black the more absolute the mask.
Let me know if that helped!
Unlike some other Robin Hoods, I can speak with an English accent!