Just tried a videochat between my Powermac G4 and a windows 7 pc.
Here they are the steps you have to do:
1) install the latest skype application for windows desktop
2) log in with your skype account on this appplication
3) without logging out, while skype is still opened, uninstall it
4) install skype 5.10.0.116 for windows, you can find it on the web
5) open it and it will automatically access to your account you logged before on the newest skype version
i know it is a long process, but it will allow you to videochat on skype and even to share your screen (a function only skype has).
if you try to log in directly from skype 5.10.0.116 it won't allow you to do that.