Simple script that will rotate between text/image ads

I need a simple script written that will rotate between my text and image ads on my site. It must work on my .NET site. I should be able to designate the ratio of which the ads gets displayed (e.g. ad #1 - %20, ad #2 - %50, ad #3 - %30). It should be very simple for me to go into the code and change up the ads and display ratio. I know how to do this in PHP (easy), but it won't work on my .NET site. If you can do this in Javascript and it works, that is fine too. As long as it works. Please send me any questions you may have. Thanks.