It depends on the reason it is blurry and the image itself. Photoshop was some limited tools but if the image is valuable then some advanced techniques such as "deconvolution" can be applied with some specialized software. But going past what PS can do basicaly requires that you find, hire or become an expert in image procesing You will need someone who knows what "Two Dimensional Fourier Transform" means. The trick is to determin and model the cause of the blur and then calculate what a geometric point so blured would look like then the deconvolution using the Fourier Transforms and its inverse becaomes "just turnning a crank". I've left out huge amounts of detail but left in enough to show that this is not just a point and click fix in Photoshop.
Also be warnned that all of these methods add noise and other artifacts to the picture and only work if there is enough data in the image file.
Fourier Transforms and deconvolution? No need to go "No Way Out" on the image yet guys
The tool to use to attempt this in Photoshop is "Unsharpen Mask" or "Smart Sharpen" (for CS2). Use repeatedly with moderate settings (70%, 1px, 1 level). Another trick is to reduce the size then sharpen again. How well either of these techniques work depends on how blurry the original is and what the intended use of the final image is. The lower resolution your final version will be (eg used on a web page vs 8x10 print) also increases your chances of success. Sometimes the image is just too blurry but trying the above might help. Good luck.