Photoshop help: Desaturating the colors on multiple layers.?

First and foremost, remember that I have version 5.0 of Photoshop.

I know that I'm supposed to make several layers of the photo I want desaturated, and I know how to highlight one specific color (Magnetic Lasso Tool around the object, and then desaturate background). What I don't know is how to highlight more than one object; It seems that I can only encompass no more than one object with the Magnetic Lasso Tool. Does anybody know how to "protect" more than one object from desaturating?

Another thing: Does anybody know how to change the Eraser so that it desaturates instead of erase (Although, I suppose you are erasing the color from the layer atop the original one)?

I saw my teacher do this, but I don't know if version 5.0 can.

Thanks in advance!

Photoshop help: Desaturating the colors on multiple layers.?anti virus protection

The way I would approach a problem like this is as follows. (I'm using Photoshop CS2 but I think it will work with v5):

Create a saturation layer (setting saturation to 0%), then hide it using a layer mask. (The mask will appear black.) Then you can "paint" with a regular, white brush directly onto the mask. The desaturation effect will appear only where you paint.

To desaturate specific objects, lasso around one at a time, make the mask active by clicking on it, then fill the selected area with white so that again the desaturate effect shows through.