I am asumming that you are aware that when you scan in a photo it is turned into a series of pixels. When viewing this images, if you zoom in too far (>100%) then you will see the pixelated image. Normally, after scanning a photo, some sharpening is required to make the image a little clearer.
To help you further, could you supply some information please.
1) What operating system and version of PSE are you using? - Can influence the answers.
2) Do you scan directly into PSE (Get from Scanner) or use an external program?
3) What is the dimensions of the photo (in inches or cms)?
4) What is the pixel size of the scanned image?
5) Does the photo look OK when you first scan it in but then gets worse after you edit it?
Also, what type of file is the scan being saved as (jpeg, tif, etc.).