Maybe it's a problem with anti-aliasing settings. Some versions of Irrlicht, together with certain video drivers, tend to render thicker lines instead of anti-aliased lines. You might want to check out a mor recent version of Irrlicht from SVN where these issues should be fixed.
my version is the 1.6, the last. the video driver is the opengl. It is sufficient to change the video driver? What type of video driver is the best in this case?
Since d3d does not do line smoothing without multi-sampling you won't see this in any driver besides OpenGL. Since the 2d material is not configurable in versions prior to 1.7 you need to recompile the engine or wait for 1.6.1. Simply set the anti-aliasing settings in the 2d render mode method to disabled.
Eh, no, that won't work. You have to change it in COpenGLDriver.cpp All other settings are simply overwritten by the driver setup code. Changing this from the app is only possible in 1.7 or later.