*UPDATE*
If you want to use latest Skype version just add following line to hosts file (C:\Windows\System32\drivers\etc):
127.0.0.1 rad.msn.com
127.0.0.1 live.rads.msn.com
127.0.0.1 ads1.msn.com
127.0.0.1 static.2mdn.net
127.0.0.1 g.msn.com
127.0.0.1 a.ads2.msads.net
127.0.0.1 b.ads2.msads.net
127.0.0.1 ac3.msn.com
Today I received a Skype update that started to show advertisements in chat windows.
This was unacceptable for me, so I decided to downgrade Skype to latest ad-free version, which seems to be 6.9.0.106.
You can download old Skype version without ads from here:
http://download.skype.com/msi/SkypeSetup_6.9.0.106.msi
Mirror:
http://mirror.szepe.net/software/SkypeSetup_6.9.0.106.msi
To prevent Skype from updating automatically go to:
Tools -> Options… -> Advanced -> Automatic updates and click “Turn off automatic updates’.