Hi PaulW,
What you are going to do is delete keys by using regedit (can start it through a dos window and type in "regedit"). Since we are messing with the registry please backup => Especially your lightning files (either online or the actual folder with notes). As a caveat you should really backup your system in case you accidently delete another key you need. With all those caveats - here is what you should do: (did I mention backing up?)
1) Start regedit - search for keys (all programs->Accessories->DOS prompt)
2) use the search dialog (under Edit->Find) to find keys associated to Lightning (will be under folders with Corel or WordPerfect)
3) Delete the key entries for Lightning
(Another caveat - if you have Mozilla's Lightning product installed - which was for calendars probably would be associate to Thunderbird installation) you would have to be careful not to delete any of their keys). If you don't know what either of these are you rpobably do not have to worry.
Once you delete the keys you can try to reinstall using the CDs. It should not be greyed out. Let me know how it works.
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On another topic for others who may read this thread. As per the disclaimer on this site. While I work for Corel, this site is not authorized or endorsed or anything else by Corel. So, I get to threads as soon as I can but mostly it is on my own time, so sometimes it does take a few weeks (for which I apologize). In this case I was OK though - phew! But, same goes for any thing I may counsel to try. I am pretty good with PCs, but would hate to do something to your system that causes you pain. So, please do backup.
Cheers,
Jay