Have you ever been frustrated at your Gtalk just because it would not send files or would not allow you to call your friend , I have been hit by thing several time until I saw the solution somewhere and I am writing the solution here and now,
Now you might be remembering these error messages in google talk,
You cannot call because "Your Friend" is using chat in Gmail or another chat program other than Google Talk.
You cannot send files because "Your Friend" is using chat in Gmail or is using another chat program.
The solution is easy and simple,
1. Logout of Gmail if it is open in your browser ( firefox,IE,safri,opera anything)
2. Now sign out of Gtalk and login again , this time you would be able to send files and call your frinds
So, the real culprit is the currently open Gmail tab/window just logout of it and logout of your Gtalk too , and relogin in Gtalk to get the things going smooth and the problem is resolved then .
Hope it helped ,