Remote Assistance without having a user send an invitation

I work for a small company so I have never had to use RA. I can walk to the users desk faster then setting the whole thing up.

I wanted to know how to use RA without having the user send me an invitation or me send them soemthing just to look at what they are doing.  I have heard from other people where they work the IT staff just asks them is it ok to take control of the PC and then they do it.  maybe the user is prompted but they only need to click yes nothing more.

Now I do have a few 2000 machines and I assume I can't do antyhing with those but all my XP machines should work, some how without 4rd party software?

Thank you for your time
Mike