Concept: The official Windows OS "Install/repair DVD" images are are available on the Internet. To get one, just download the appropriate version, verify its checksum, burn it to DVD-media, and you are all set. It's perfectly legal and the kits are FREE. [ They're free because back when you "purchased" (e.g. as part of the cost of the computer) the copy of Windows OS that came pre-installed, what you actually bought was a unique 25-hex-digits-long 'product KEY' (aka an 'activation-key'). That key is what gets registered at Microsoft (via an 'activation' Internet transaction). So if your pre-installed copy ever gets damaged (e.g. due to hard-drive failure or severe spyware/virus corruption or whatever), rather than needing to use the kit to do "repair/recovery" you can give-up on the repair/recovery approach, and (re)install the Windows-OS yourself. Just try to find/jot-down that 25 digit product key (usually on a tag on the top or front of your computer), and re-enter it at the end of the installation procedure. But, if that tag is blury or missing, you could download/install a neat utility called 'Belarc Advisor', which can fetch the product-key from the registry, assuming the original drive is in working order. If any of those methods are unworkable, complete the installation, but bypass the step where they ask you for an activation-key...instead try calling Microsoft support on the phone, and ask them to (please) activate your installation for you, (using a key they'll supply, if they're satisfied with your explanation of why you don't have the existing key. NOTE: I believe this works for all of Windows-XP, Windows-Vista, Windows-7, Windows-8.x, and Windows-10! (I've called support and they have activated me on two or three separate occasions. My 'rationale' was that I was legitimately installing a different VERSION of Windows, because I had signed up to 'beta-test' pre-production edition of Windows-10, and had severe problems, and had wanted to drop back to my original Windows-8.1. ]
OK? So, here is such an official website, where you CAN download a copy of the any of the various Win-7 installation image-files (do a google-search to find images for other Windows versions). Make sure that you get the matching flavor of the kit that was originally installed/registered for the computer in question: e.g. "Windows-7 Home Premium, English, 64-bit" edition (or whatever). Note also that these installation-kits often contain and auto-apply existing service-pack(s) (e.g. 'SP1'), which saves you time later, when you update the resulting installation to most-current patch-level, via 'Windows Update'.
Win-7 direct downloads[ Alternatively, there's the concept of using a 'recovery' image, which is explained here: ]
Recovery-image method: (How-to article)As an example: On any WORKING computer, download a free Windows 7 ISO image file and put it onto a flash-drive (or burn it to a blank DVD). Such an image/kit can be used to both repair an existing Win-7 installation or to install Windows from scratch.
Also, you may need to download the OEM-supplied drivers for some devices (because Microsoft's kit doesn't always contain all the needed driver(s), e.g. for the wifi-adapter, or the audio-driver, etc. So, you need to visit the appropriate OEM's support-website (e.g. Compaq or Dell or Lenovo or Gateway, or whatever, based on the make/model of your computer, or those hardware devices. Then, use Window's "device manager" (it's in Windows' Control-panel), find the specific device, and choose 'Re-install driver'.