Finally, I threw my PC away and now there is no PCs in my room. The reason why I bought this iMac is my old desktop was very very slow!! It took me 5 minutes for me to turn on till it was really for use. It is a common fault of Microsoft Windows. Your windows would becomes slower slower and slower.
But it doesn't happen in Mac. Now, it takes 40 seconds to boot up till it is ready to use. It is brilliant !