But, if you really do want to try to back up your entire hard drive, you can use this:
xcopy C:\ E:\ /e /h /k /c /d /y
This assumes that your hard drive is C:, and your external USB is E: and that you want to back it up to the root of the drive. This will not back every single file, but it should get all files that are not in use (files like pagefile.sys, registry hives, etc. won't be backed up with this method). It is suitable for backing up your data files, documents, pictures, etc., but you won't be able to use it to restore your entire hard drive.
Here is what the switches do if you are interested:
/e: copy all subdirectories including empty
/h: copy hidden and system files also
/k: keep the hidden / system file attributes on the copied files
/c: continue copying if an error is encountered (like a file is in use)
/d: copy only newer / changed files
/y: overwrite existing files
Notes: the /d and /y aren't necessary if this is a 1-time copy, but if you run the command on a regular basis, they will significantly speed up the backup because xcopy will only copy files that have changed or don't exist on your destination. Also, xcopy has limits on paths and file names and performing an xcopy on an entire hard drive will sometimes result in an "out of memory" error (usually in extremely long paths or filenames). I get around this by using the robocopy program which is a free download from Microsoft.
If you want a full backup, I would go with something like Acronis as suggested by Patio ... although I do have to respectfully disagree with his opinion that backing up to CD/DVD is better than an external hard drive. All storage media is prone to failure, and all storage media will fail eventually. I personally prefer an external drive over CD or DVD media.