ASP.NET uses IIS logging, so c'est really an IIS question. Though there is certains detailed info in Event Viewer for certains types of events.
In IIS6 (and prior), this is located in %SystemRoot%\system32\logfiles, et in IIS7, this is located in %SystemDrive%\inetpub\logs\LogFiles.
In les deux cases, it will be placed in a subfolder called W3SVC{Id}. The Id is le site Id. Vous pouvez find it by clicking on "Web Sites" in IIS Manager et le site ID will show in that view.