Finally I have solved this problem ! It has been caused by corrupted DRM caches.
At first, open the url shown below.
http://www.universaldigitalcopy.com/support/support_application.html
Next, follow the instructions from 1 to 4 in the section "When I try to obtain a license, why do I get a continuous loop of prompts asking me to upgrade Windows Media Player or Windows Media Security?". I have quoted these instructions in case of trouble to see the webpage.
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begin quot -
1. Close Windows Media Player, then go to My Computer, then to the C:\Documents and Settings\All Users directory. (Vista users, go to C:\ProgramData\Microsoft\Windows) If you don't see this, that's because it's a hidden folder. See below for instructions on how to set your folder view options to make this folder visible.
2. In this folder there is a folder called "DRM". If you don't see this, that's because it's a hidden folder. See below for instructions on how to set your folder view options to make this folder visible.
3. Select the DRM folder, and select File > Rename from the toolbar menu at top.
4. Rename the folder and hit Enter when done.
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end quot -
Now run WMP and open the file that has been protected by Windows DRM. In my case, WMP said something like "some components must be upgraded" ( I am using Japanese version of Windows XP). Click OK and then WMP proceedsto retain license from the Internet. After all these instructions, WMP plays media files normally again.
