IIS Express is a developer tool for Windows XP and higher which provides the full feature set of IIS, but without needing administrator rights.
I've seen discussion by some developers who are considering bundling this with their application to make deployment consistent among versions.
If you read the comments section in the link above, the blogger ScottG (and MSFT Corporate VP) mentions that instances of IIS Express will not receive updates via Windows update, but they might be updated when using Microsoft Update.
Since having unexpected and unpatched deployments of IIS Express in the environment of the application end-user can be an attack vector, how does their IT Security organization audit and detect the usage of this product?