For sharing presentations and applications on the desktop you can use TeamViewer.
A browser and a telephone are enough to take part in such a Webinar: http://www.teamviewer.com/
Start the program and follow the Steps as described in the Handout (Support\First Steps): English, Deutsch
An e-mail will be generated with the login data, which can be sent to everybody who shall attend the conference.
PowerPoint 2003 training courses - online
A roadmap to PowerPoint 2003 training:
English, Italiano, Francais, Nederlands, Deutsch