Try checking all these steps: Launcher Setup Errors/Bugs and their fixes - CHECK HERE BEFORE POSTING A BUG/ERROR
If none of that helps try to manually delete all remaining files there are. You should find them on the Harddrive you installed it on e.g.:
- C:\Program Files (x86)\C&C
- Documents\my games\Cattle and Crops
Make sure to delete these folders before trying to install the game again

I don't know if you read whole thread posted by @TheCoCe.

This one below usually helped to several mates.

You told, you have had at least once Tech Demo installed. I would say, that you forgot for something to delete manually and that's why it doesn't want to go on.
Shortly, check those quoted paths.
We can go calmly step by step.
1) you have probably done this step - uninstalled over control pannels.
2) This one you might overlook and some old datas might stay in this folder (that's why it doesn't work).
3) Probably done.
4) Dtto.
5) Try it again - download launcher, start it (by launcher.exe or how it names) from default folder, or designated by you. The launcher tab should appear. Then connect by your access name and password and click to instal, which should appear in the right -middle, or click to left top button in the launcher tab, and click install/reinstall.

Write if it helped.