Advertising System

I need a central advertising system where advertisers can buy ad spots to all my web sites. There should be Global and Category based ad spots, where global ad spots are shown on all pages of the website and Category ads shown only in a specific category. The Ad system should be complete with a clean web 2.0 design, mysql databases and php code (preferable in OOP). This is what advertisers should be able to do: register a simple advertising account. login/logout. after login you should have a main page where you should be able to see your current ad spots, see statistics for your ad spots(clicks and pageviews per day/month). choose which website to advertise on. choose advertising spots on selected website. Payments should be through PayPal. all transactions should be logged in database. extend advertising spots. Make it simple but powerfull, its very important that the system is very easy to understand and use. It should be delivered with design, tested and bug-free.