File.Delete cannot delete files

When calling a function a() with a thread, there is File.Delete in a(), and "Access to XXX path is denied" appears,

but it is completed correctly when there is no thread.

After searching Google for a long time, I found "Add IIS_WPG user, IWAM_#### user, ASPNET user, ISUR_#### user"
necessary permissions".

ps: Windows 2003

Source: http://coos.cnblogs.com/archive/2006/05/15/400507.html