Buy another Internal Hard Drive (HDD). Unplug your power cord, hold the power button in for 1 minute to discharge any remaining electricity. Be sure you are grounded (wrap a wire around your wrist and attach the other end to something metal other than the computer). Reach inside and carefully move the wires a little to see what type of connection you have to your main hard drive (sata = one thin wire) (ata = a ribbon type cable). If you find you have SATA, follow that wire to the motherboard to see if there are 3 or 4 more SATA connectors. If so, any new SATA hard drive such as Western Digital, Hatachi, Maxtor etc. will cost you about 80 to 100 dollars for a 500 to 1000 GB hard drive (1000 GB = 1 Terabyte). Look for a bay above or below your main hard drive to make certain you have room for another Internal Hard Drive. If not, then go with a good External Hard Drive (same mfg/s plus others). Your transfer rate for backups and system images is much faster with two Internal HDD's. Here you can see what mine look like in Windows 7 32 Bit (Windows 7 classic mode).
Choose Windows Backup Utility (if you have it) or download any number of hard drive backup software programs. A very popular free one is at Cnet.com called
Macrium Reflect. I happen to use a hard drive manager program that allows me to do many things with my hard drives. Most users never need a sophisticated program. Try Macrium, I think you'll be happy with that one. I'm sure others will suggest a couple other freebies as well.