And a big part of those 20% are for corporative computers, that will not change it too easily, unfortunately. Because my new site is not working with IE6 and I missing those traffic.

Don't give up! There is usually a solution to display bugs. For instance, there's a float bug that can be solved with a few simple tweaks [webmasterworld.com]. I know HTML purists are going to get upset at this, but sometimes changing the DocType to 4.0 Loose can cause it to start displaying right, but check it against all other major browsers because that can introduce other errors.

The major point here is that it's easy to forget that IE6 is still a major browser and you owe it to yourself to check what your site looks like on that browser because it could be the cause of depressed earnings.

It seems to me the problem is with AS not being capable of delivering ads compatible with a large percentage of browsers.

I've made sites using XHTML and had no problems. More likely errors in coding that's causing the problem.