Although the reason caused this problem is still unkown...I should be aware of that in case it might happen again.

Steps:

1. Put the Windows installation CD into DVDRom then restart.
2. Let it scan the PC, then choose R to recover.
3. Choose to identify the existing Windows system.
4. Or type in:
   C:\BOOTCFG \LIST to list all booting system
   C:\BOOTCFG \ADD to add one booting system or
   C:\BOOTCFG \REBUILD to rebuild the booting .ini file.
5. Note that the BOOTCFG "Identifier" is the identifier like "Windows XP Professional".

Now restart and enjoy.