I use NOD smart security  and it always alarm me my computer is affected by win32/TrojanDownloader.Wigon.BS and the infected files are isolated.
I've scanned for seversal times by smart security.  How can I remove this virus?

I have just checked NOD32's virus database and found that the virus you mentioned above had already been identified by NOD32.o
Please try to do a full computer scan in Windows Safe Mode and see if the virus can be removed.

Or maybe you can try to use ESET SysInspector to analyze if there's any suspicious programs running on your PC: