Did you ever thought about the folder and files created by Windows XP for the ASPNET user account?
"The ASPNET user account is an internal account and no one is able to log using this identity"
This is the most common answer for my question. And yes, the answer is true. At least it touches the most important part. But there's more.
You see, since Windows XP invokes ASP.NET processes using ASPNET account, it also automatically creates a user folder with typical structure (Application Data, Cookies, My Documents etc.) for this account.
And here comes the strange part: the folder structure for the ASPNET user account is created under "Documents and Settings" but instead of username as a folder name, the machinename/username is used.
Suppose now that your machine's name is XYZ and you use a user account named XYZ. Guess what - you'll see ASPNET user local folder inside your own user folder.
Something tells me that this is not quite right. I should not be able to peek into other user's files but since the folder sits inside my own local folder, I am able to browse it with no restrictions.
I would say that this is rather insecure, no matter if it can be easily misused or not.