There's various ways to start again.
You can either select to uninstal after starting the original Hoster5.06 setup.exe file.
Then delete all your song files of the hard drive. Then reinstall, if the tick box for clearing all previous settings shows then tick it and install this should then be as a new installation.
Another way, that will allow you to select what to remove is open the Tools/songs database then the build database, in the dialog that opens select all the folders in the list of folders that you want to remove and press the "Exclude Folders" button.
This way you can either select all the folders or select which ones you want to remove.
That should clear all the songs that you don't want from the database but to be sure I would use the rebuild database button, with no songs in the list of folders that should build an empty database or one with just the folders you left in the list.
Then if you want to remove all evidence from the songs_backup.mdb either rebuild it again that will back it up the same as the Songs.mdb one, or delete it from the C:/MTU/Hoster folder.
Then if you want to remove the original files from your hard Drive completely use windows explorer and delete the folders that way.
If you decide to keep all your previous files and try to import them again especially your CD imported kma files after importing all these new files, you could run into trouble with duplicate book id's or disc id's.
Also you will probably have to delete your singers.mdb as all the pointers would be wrong.