In addition to what Brink said, in most cases when you install a program, it is not only copying files to a location under Program Files - it is also usually copying files to either somewhere in the windows tree (DLLs) or else somewhere in the users tree (Settings files). Without those in place there is a slightly better than even chance that your app simply will not work.
Even worse, though, is that almost *all* program write info to the registry, and those are usually more required for operation than the files can be - usually without the appropriate reg entries about 75% of programs will fail.
Therefore, the best bet is to simply re-install them.