Thanks.
I used Start -> All Programs -> Accessories -> System Tools -> System Restore as you suggested but was not successful. After I run System Restore, I got the following message:
System Restore did not complete successfully. Your computer’s systems files and setting were not changed.
System Restore could not access a file. This is probably because an anti-virus program is running on the computer. Temporarily disable your anti-virus program and retry System Restore.
An unspecified error occurred during System Restore (0X80070005).
You can try System Restore again and choose a different restore point. If you continue to see this error, you can try an advanced recovery method. For more information, see what is Recovery.
I tried to choose a different restore point, it still have the same error. I do not know what is an advanced recovery method.