When I start my windows it opens but the window cut down. My monitor is 17" wide, but it appears 15". When I restart it , then it comes perfectly. I am sure that there is no virus problem, because I have set up windows XP again. But this problem was not solved. Can any one say actually what is the problem?

First thing that came to mind, could be a display driver issue. Did you update it?

No . I did not update it. How can I do it?

Okay, first open Device manager and look for it under Display.

If it comes up with this
Standard VGA Video Adapter or some sort (sorry forgot the real one), check your motherboard model.

After you got the motherboard model, search it with google along with the windows version you're using now...

First try set the screen resolution in desktop properties.

First try set the screen resolution in desktop properties.

It may be not the screen resolution problem because when I restart the PC it open perfectly.

More the reason.
If you just fresh install operating system (mostly Windows OS's), they will use the basic driver to run so it can't run some application and have problems with display and any other devices.

To correct this, you must update all your driver. You can use any batch free driver checker and install for easier method or search it manually which may take more time to install them all.