Hi Hafsa,
I'm not sure if you can perform desktop sharing without installing some kind of software. There's a free web service that allows you to transmit your desktop view (live screencasting) over the web and also includes an interactive WhiteBoard:
http://www.tokbox.com/ It uses open protocols and is friendly to Jabber, GoogleTalk, YIM, AIM, etc. but not Skype. Skype is a very closed protocol so you can only really do Skype to Skype stuff with it.
There isn't a Moodle integration available for it though. There really should be.
Another option is DimDim. They have a Moodle integration module but I haven't been able to find sufficient documentation to get it to work. If you do, please let us know!
I hope this helps.
Matt