Hi. I've been experiencing this problem for quite a long period of time,but now I am really sick and tired of it. My first Gl-engine(which originally was just a small NeHe's Lesson1 - based test app,and now is a huge monster of nearly 500Kb) not always switches to fullscreen mode when try to run it in fullscreen.Well,actually it switches to fullscreen,but the task bar remains still visible for about 5 secs,and framerate is also lower. How can I pursue it to run in fullscreen for sure?

Lower framerate is because vsync is enabled (framerate is at most your refreshrate).

The taskbar problem is just a windows thing, iirc it hangs for some time when going to fullscreen, probably because of windows needing to resize or something.
AAA game have it too, halflife 2 has it, farcry has it, every fullscreen game has it, some games more than others. I don't know how to fix it but it shouldn't be much of a problem.