First, you'd want to run a software exam called the Upgrade Advisor to see whether your machine has enough memory and computing power to handle Windows 7. It's available here: http://bit.ly/2eljBW.

Most computers that are running Windows Vista should be fine for Windows 7.

If your PC passes the test, you'd need to choose a version of Windows 7. You would pick from Home Premium (US$120), Professional (US$200) or Ultimate (US$220). (Big businesses can also buy an even more feature-laden version, Enterprise.)

For most people, Windows Home Premium will be fine. Microsoft will sell the software as a download on its web store, but copies on discs are stocked at electronics retailers and available from such sites as Amazon.com.

Make sure you buy a copy that's compatible with your PC's microprocessor. There are two flavours, 32-bit and 64-bit. Microsoft offers help in figuring out which one is right for you: http://bit.ly/3qPP23.