There's a few options, and it really depends how you want to use it.
If you just finished to install an OS (I assume it's a windows XP family) and you want to keep a copy in case something goes wrong, than Acronis True Image can save the image onto a DVD (or a few CDs), and restore it in minutes.
If you just installed something and want to copy to a different hard drive, or if you bought a new bigger hard drive and want to copy the old drive as is to the bigger drive, the XXCLONE (freeware version) is a great choice.
For monthly backups (I would say daily, or weekly, but it doesn't really happen...), Acronis True Image can also make a snapshot (again, it might take a DVD or CDs).
If you have a second hard drive or another partition on the hard drive, you can use the XXCLONE. But keep in mind, that XXCLONE will delete everything that you have in the 2nd hard drive, and make it a clone of the first.
Keep in mind, that a backup strategy needs careful planning, and testing (yes, test to see if your backup really is worth it. I know many people that use tape backups for years, only to discover when disaster stroke, that it didn't really work).