Make sure you get a backup software. There are tons on internet, choose what suits your demands. I personally use www.safecopybackup.com. If want to give a try, they have 3GB free trial. You'll like it.

Do u have a new one ready to replace? What's wrong with the old HD? Note that your hard drive may fail and u lose data. Free advice, backup up ur important data online for safety.so u can access it even after any disaster. checkout at www.safecopybackup.com. They have 3GB for free trial and never expires.I personally use it and it's been good.