Expanding banner ads - help!

Hi,

Can anybody explain or help me understand how to create expanding ads?

Obviously I can create a movie file which expands and retracts- but need to know the proper way of doing these so they will work when hosted on my clients site.

The ad must expand over the html content, and have the option to ‘close’ down to a small button.

I’ve checked out tangozebra.co.uk who seem to have an application to build these - but with the easter break i can’t contact them!

Any help would be much appreciated. I could even pay if someone knows how to do this.

Thanks!