It may sound too good to be true, but on a typical Windows computer you have everything to do automated backups to an external harddrive: XCOPY and Scheduled Tasks.
Xcopy documentation and examples: http://support.microsoft.com/kb/289483
Scheduled Tasks: http://windows.microsoft.com/en-us/windows7/schedule-a-task
xcopy can be configured to copy files from a source to a destination only if they changed or are newer
, and this can be put into a batch file which is then periodically called by a scheduled task. Alternatively, you can also just manually start this file whenever you like.
By limiting the copying to new or updated files, the backup in most cases take a lot less time than just blindly copying everything each time you're starting the backup process.
Depending on which version of Windows you are using, the specifics differ a little bit, but you'll probably find quite a lot of documentation for each version online. Here's an example for WinXP: http://www.ekho.com/Training_Videos/XCOPY_NOTES.pdf
I have been using xcopy on Windows for years and it's a unsophisticated but reliable tool for simple cases which include no need for revisioning or transfer over networks etc.