How can I make the enter key on my extended keyboard act as return only? For some reason sometimes the numeric enter key acts as return and sometimes it doesn't. (I'm talking of the last key on the keyboard, the one with the "k" symbol tilted towards left.)

I'd like to be able to use it as the regular enter.

Do you know any tip in order to fix this? At the moment it is totally random. My guess is that some software  I use remaps the enter into return, but I've not been able to point to which one.
You've got your assumption a tiny bit mixed up. It's not that some software remaps the enter into return, it's that some software treats an enter as a carriage return and some doesn't.

They are two different keys with two differnet histories. One is enter and one is carriage return and they perform different functions.

I hate referencing wikipedia but hell, here ya go ... Differences between Enter and Return.

Usually most programs assume they are the same key - some don't. The AppleScript Editor for example uses &#x2318;+enter to force a script parse.