Are you working with a bitmap?
If so, the quickest way to select a rectangular area enclosed by guides is to use the Marquee tool. It will snap to the guides if that option is selected in the View > Guides menu.
Once the selection is made, you can either crop using Edit > Crop Selected Bitmap or fill with color using the Paint Bucket tool (choosing Fill Selection in the Properties Inspector). Actually, another cool way to fill with color is to choose Select > Convert Marquee to Path. This will create a vector object based on the selection, to which you can color, add texture, etc. in the Properties Inspector.
Another way to create a rectangular marquee selection would be to draw a rectangle and then choose Modify > Convert Path to Marquee.
