Automatically adding a banner to a site

My web hosting account permits unlimited domains, and gives generous bandwidth, so I host some domains for free. Basically these are work colleague's and friends sites.

Is there an easy way to place a banner add on their sites? Either an anoying popup window whenever their hompage is viewed (OK I dont like that idea), or better, at the bottom of each site page a small add placed - similar to Geocities? I realise that this would need to be done at the server side of things as I could hardly ask the people to add the advert themselves!

PS (I'm on a windows server).

Thanks for any ideas or suggestions.

Nathan

I've done something like this with apache and I know it can be done with iis, but don't know how. Basically though, you would have to change iis settings to inject the code. Don't know if a shared host will be willing to do that. Would have to be done for each domain/application.

There's some question about how/if some of the search engines treat links from the same ip. Some people think they don't count, which makes sense to me as there'd be nothing preventing me from puttin up 1000 domains/subdomains with links to my site. But I've never seen any test results one way or the other.