Let’s say you’ve had Lightroom and Photoshop CS2/CS3/CS4 working happily side by side on your Windows machine, and you decide to upgrade to CS5. Now Lightroom sees CS5 as the primary external editor and you proceed happily on your way. Time goes by and you come to the conclusion that you love CS5 so much that there is no point keeping the older version of Photoshop on your machine, so you uninstall CS2/CS3/CS4.
All seems well until you launch Lightroom the next time and see Edit in Adobe Photoshop (not found).
But you know CS5 is installed because you can launch it directly just fine! So, thinking this is a Lightroom problem you might trash your Lightroom preferences or uninstall/reinstall Lightroom to no success. The issue is that Lightroom relies on a registry setting created by Photoshop when it is installed. However, when you uninstall the older version of Photoshop it removes that registry setting and Lightroom becomes blind to Photoshop’s existence.
Adobe has a TechNote that provides 2 solutions to fixing the problem. Both solutions work. Most people seem more comfortable with the reinstall of Photoshop option over the registry edit. Unfortunately, the only way I know to avoid the problem is to uninstall the old version first, then install the new. Hope that helps you plan ahead or at least regain your sanity after discovering the problem the old fashioned way (by accident).