in some cases if you delete the main BFV folder you also have to eliminate some registry keys.
just run regedit and put in search battlefield vietnam, it should come up with a few results even if you deleted the main folder. there are no windows keys called battlefield vietnam so you can delete all that you find
if you look for BFV as well, then be careful. usually the files you need to delete are in a folder called EA or battlefield vietnam, or something like that. if you find it in a windows system folder or some crap like that dont touch it by all means. tinkering around in the registry for the first time takes a bit of balls.