How can I back up my entire C drive?

I’m trying to back up my entire C drive. But I keep getting “Such-and-such file is in use. Try closing any open programs and try again.” It won’t just skip over the offending file, it just stops the backup cold. I’ve closed everything, and still get the error. I tried copying one folder at a time from the root directory, and still get the error. There are thousands upon thousands of files and sub-folders, and I can’t possibly copy them all one-at-a-time. Help!