Many thanks for your very detailed information, Lorien, I created a new account then, but the problem still exists in the new account's start menu. And I also scanned my computer, no trojan were found. Unfortunately I disabled the system restore point long ago.
This problem happened just in this afternoon, I scanned my registry problems with two programs which were called "wise registry cleaner" and "RegistryBooster", (I did this because I got another 'media player parameter incorrect' error), mean while, I plugged in my mobile phone to transfer something, the task-bar then froze (probably because the system wanted to set up driver), and I couldn't activate the task-bar, so I decided to terminate the 'explorer.exe' process, but I couldn't restart 'explorer.exe' anymore, I tried many times, then I have to turn off the computer by force pressing the power button. The next time I restart my computer, the Personal folder was missing, and I still bear the 'media player parameter incorrect' error. Before I did this, the start-menu works fine, so I guess this might be caused by the registry, I guess there might be a key in the registry controls the Personal folder link in the start menu, another guess is my explorer.exe might be crashed.
Best regards.