First of all, I love Lightroom...when it is working.
I have downloaded all of the updates and followed the troubleshooting provided by Adobe... I have uninstalled it and reinstalled it. Same issue...when I open it, it says "Lightroom encountered an error when reading from its preview cache and needs to quit. Lightroom will attempt to fix this problem the next time it launches."
In the folder containing your LR catalog there is also a folder called Previews.lrdata (with the catalog name in front of Previews).
Delete this folder - don't delete anything else. (the .lrcat file is your catalog, don't delete that!)
Lightroom should now work, and will rebuild all the previews.