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It all depends on the Make and Model of your Computer. Let me know your Make and Model and I'll let you know if you can and how much it will cost.
The quickest way to tell for sure is to run the Windows 7 Upgrade Advisor from here: http://www.microsoft.com/download/en/details.aspx?id=20. Download and install it. It will scan all of your Hardware and let you know if your computer is fully compatible. Windows 7 Home Premium is the most commonly used version of Windows 7 and runs $119.99 from Microsoft here: http://windows.microsoft.com/en-US/Windows/shop/windows-7/home-premium. Hope this helps my friend.