Desktop 2.5 Won't Let me Upgrade to Plus

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Having a problem upgrading my Desktop 2.5 to Plus. I've ordered and obtained a valid key, but when I enter the key the upgrade process aborts with the message

Error: Required software component is missing

I've tried to the following, to no avail

1. rebooting
2. uninstalling and reinstalling 2.5
3. downloading a fresh 2.5 file from Tivo and installing
4. reinstalling over the existing 2.5 so it goes into "repair" mode
5. updating my Java to the latest version

Does anyone have any ideas?

My initial install was "clean". (I originally had problems with it and had to completely remove the earlier version of Desktop and edit the registry to remove all traces of TiVo before reinstalling; see other threads for more on this problem.)

Every other aspect of Desktop 2.5 seems to work fine, it just won't let me upgrade to Plus. I'm certain the key is good, because I entered in an invalid one as a test and got a message indicating the key is invalid.

Thanks in advance
.