When I try to open Lightroom I get an error message telling me that "the lightroom catalog names Lightroom 5 Catalog cannot be opened becase another application already has it open"
when I choose a different catalog there's only one catalog available.
If your catalog won’t open, check for a ‘your catalog name.lrcat.lock’ file alongside your catalog file, and delete it, and then try to restart. The lock file can get left behind if LIGHTROOM crashes, preventing you from opening the catalog. If you find a ‘your catalog name.lrcat-journal’ file, do not delete that as it contains important information.
Re-installing Lightroom will not delete a ".lock" file.