Can Not install any apps

I have a macbook 2.1ghz, with Mac os x 10.6.1 My problem is that when ever i try to add an application to the app folder to install it, it says that it is

"An item named “xxx” already exists in this location. Do you want to replace it with the one you’re moving?"

after selecting replace i get

"The operation can’t be completed because an item with the name “xxx” already exists."

but this program has never been installed before so i dont know why it would already be there?

Which application are you trying to install? And does the same occur when trying to install other applications?

Regards.

I am trying to install speed download, but this happens with any app.