1. Open a new image 10x10 with a transparent background. 
2. Choose View> Zoom in by 5. 
3. Make sure you have rulers on ( View > Rulers). 
4. Select the Draw tool and in the Tool Options palette choose Point to Point, width set to 2, Antialias checked, Create as Vector and Close Path unchecked. 
5. Set foreground color to black and turn off background color. 
6. On your image do a point to point line both horizontal and vertical at the midway point on your image ( 5 on the ruler ) from one edge of your image to the other. 
7. What you will end up with is the plus ( + ) sign. DO NOT MERGE AND FLATTEN!
8. Save as a PSP image for later use but don't close the image. 
9. Open up a new image that you want to put a transparent grid on, add a new layer and flood fill with your grid pattern (this will be a pattern fill selected from patterns in the styles palette ). 

This will give you a transparent grid over your image. You can also change the grid size by changing scale in the pattern palette making the grid either larger or smaller. You can even change the color of your grid by selecting the solid line on the grid [may want to zoom up for this] with the magic wand and then flood fill with the color of your choice.