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Hello and thank you for your question. I look forward to assisting you today. When you replace the hard drive, you will need a new installation disc for Windows. Unfortunately that means you will need to purchase it through a retailer then install it. You may be able to connect to old drive to your computer using an adapter to copy your old files depending on the condition of the drive.
You can continue to use Windows 7 if you like or upgrade to Windows 10? The choice is yours. Which would you like to use?
Do you have your original Windows activation key?
If you have your key, then all you would need is the installation CD and you can find then at this LINK.