If you are running Skype v3 or later, there is a registry change (provided by Skype for Universities etc.) that reportedly prevents your PC becoming a supernode. I say reportedly, because whilst I have been a guinea pig for the last week I haven’t noticed being a supernode, but of course Skype may not have tried. If you aren’t running the latest version of Skype yet you should probably upgrade (Help, Check for updates), because later versions allow you to edit previous messages, and voice quality is improved.
If you think that being a supernode may cause you problems, try editing the registry as shown in the code below. (WordPress security doesn’t allow this file to be uploaded). You will need to reboot your PC for it to take effect.
This is the edit:

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Skype]
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Skype\Phone]
"DisableSupernode"=dword:00000001