The thing about that is, there even is some installation files I
already uninstalled. And some I can downloaded from the internet easily
so I don't need to waste my disk by keeping the them.
Check the property dialog of installation files (.msi) and delete them
if you don't use that program any more. Even though they're belong to
application's you currently using, I think you can remove it sefely.
But when you chooose 'add/remove feature' or 'repair' for that
application, you may need to redownload the installation file.