Finally, folder 3721 disappears. Hope it won't come back tomorrow. At least it did not show up after I restart my computer.
It is rather strange. I deleted many stuff relating to 3721. The following, however, can not be removed.
HKEY_LOCAL_MACHINE\system\classes\currentcontrolset\enum\root\legacy_cnsminkp
Any suggestion is welcomed. Thanks for those who helped me earlier.