Hi all, IIS has a bug!!
If you are trying to use IBM.Data.Informix (Client 2.81 TC3, 2.9, 3.5, etc.) from IIS, and you still have this problem, I have a solution. I found that IIS 5.1 has a bug!!. IIS cannot read the entire PATH environment variable if it’s too long (my PATH has 1364 characters). So if your informix dir is near the end of your PATH and your PATH is too long, IIS will never read your informix dir ("C:\Program Files\IBM\Informix\Client-SDK\bin"). Just put your informix dir at the beginning of your PATH variable, and very important execute an "iisreset" and the issue will gone away. I don't know if this bug remains in IIS 6.0 or later. But you can try this.
If I'm correct this will help to solve other problems when you're trying to access unamanaged dlls from IIS:
"Unable to load DLL 'foo.dll': The specified module could not be found."