Buy Acronis True Image or Norton Ghost and install. Once a day (or schedule it) have it take an image of your entire HDD and store it on the external. All set.
This is assuming you want to be able to just reimage your primary drive if something happens. If all you want to do is backup files\folders, this may be overkill. If you are running XP Pro, you can go to Start Menu - All Programs - Accessories - System Tools - Backup and run NTBackup, which is free and will work fine for what you are doing - just choose the files\folders you want backed up, choose a schedule, and click finish.
Tim