The key is likely that activation key or CDKEY. Windows isn't free and what you describe seems to be 'activation.' Here's an article about it -> http://en.wikipedia.org/wiki/Windows_Product_Activation
"I would like ask why it was happened, how I can solve this"
I think we covered that above and the solution is to give it a good valid key.
Hi! I would like to ask something about VMWare. I' ve got a macbook and I added the VMWare Fusion in order to have the windows when I need some applications that mac os doesn't support. It was ok until someday when I try to access Windows XP a message appeared :"This copy of windows must be activated with Microsoft before you can log on. Do you want to activate Windows now?" I click on "YES" and then it requires another key and I haven't. I would like ask why it was happened, how I can solve this. Furthermore I look for any option for doing format in Windows and I don't find anything. If I don't want to have the windows how can I get rid of them? I am novice to mac os.
Thanks in advance!