How to read and write file without UAC in win7 with C#

With UAC, just because you are logged in as a user with administrator privileges does not mean a program actually runs with them. The program has to either request privilege elevation (which gives the "Do you want to allow this program to _" UAC pop-up) or explicitly be run as administrator (right-click on the program/shortcut and choose Run as Administrator).

By default most areas of the drive outside of the standard "user files" (a.k.a. My Documents & co.) are restricted to administrator-only access, but the folder permissions can be changed to add access for a particular user if desired.