With new upgraded skype 7.33 it is difficult to disable ads from displaying in chat windows. Here I am sharing how one can remove it from displaying:
Step 1 : Exit Skype instance if it is running.
Step 2: Go to the below file location: See the screen shot below:
C:\Users\[system Username]\AppData\Roaming\Skype\[Skype username].
Step 3: Look for config.xml file. Open config.xml and search for the node "AdvertPlaceholder".
Step 4: Remove that particular node from the file and save it. Now if you login to skype it will be free from advertisements.