Enlarge the image to 100% full size by scrolling the mouse wheel or by pressing Control/Alternate/0 all at once. Resizing the image introduces some unsharpness in the small image which can be corrected using the Unsharp Mask tool found (moved for PSE6 - earlier versions had it under Filter) in the Enhance menu. The settings used for Unsharp Mask are a matter of personal preference and individual image situations. I frequently prefer to apply a weak setting like 40 two or three times rather than one pass at a highter number. I do prefer to keep the Radius at 0.3 and Threshold at 0 but there are books written on how to get the most out of sharpening tools and each can decide what they like. Be sure to evaluate results at 100% size (the first step on this page) or you may not see what you will actually get.