I would say that the trick is NOT to uninstall the old version of WD as it may delete registry and .ini settings and possibly your data - especially if you say you have deleted folders as well.
Any new version should just be installed over the top of what is already there and any changes to programs will replace the original ones and the registry entries and data/initial settings are kept.
I don’t think the FAQ mentions to uninstall first does it? But also it doesn’t tell you not to do this I think.
But you say a reinstall of 10.20p does not work. Maybe there is something from the old installation still there.
Somehow I think you may have to start from scratch again.
Have a read of this topic to see if it helps
Cheers
Martyn