How do I back up on a Hard Drive....

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I am going to get an external 1TG hard drive to use to back up my PC. I have apparently lost my Installation Disk for WIndows XP..( had them in the pouch with other disks, but of course, those "other" ones are still there but the installation disks are gone!) So..my question is... What and how do I back up my PC...everything on it including OS Win XP? Do I just copy and paste the C: drive? Where do I find the OS installation and all? And if I do have a crash, how do I use this info on the hard drive, to re-install it to my PC? Thanks for any information and help!