I've just installed Elements 10. Then I opened Lightroom 3. Every photo have every keyword. How did that happen? Are the 2 incidents connected? Can I rebuild my catalogue as it was with only the correct keywords attached to the correct photos?
Make sure you have Auto-Sync turned off when adding keywords... That is my best guess as to what happened.
There should be no connection to installing Photoshop Elements. Although Elements has the Organizer, which is similar to the Lightroom Library module, they each have separate catalog files. You can convert a Elements catalog to Lightroom, but this would be a separate catalog from your original and does not happen automatically.
You may be able to "Undo" the keywords, but it depends on what else you have done since then. The only way to restore the old settings beyond that would be to restore a backup copy of the catalog, if you should have one.