This question is a little similar to something I have answered before. In my experience, if you have a computer with a COA sticker on the bottom, you can activate that key for the version of Windows that it's for on that computer as many times as you want. If you don't have a Windows installation disc for that specific version of windows, you can search the internet for an ISO image and burn a copy if you have to perform a clean install.
I always try to get an ISO file from a Windows source such as This first. If that for some reason doesn't work you can always try sources other than Microsoft such as This. If you try this link, please make sure to watch carefully when you are downloading. I had to create a quick and free account, and also there is a box that you need to uncheck that ask to download with a manager.
Just please read and look at all comments about the site you are downloading from. As a last resort you can also try to search a torrent site for a non activated copy. Make sure that it is a legal copy of Windows and does not say ANYTHING about pre-activated or cracked copy or anything similar. You want a copy that you have to activate.
After you have your ISO image file you can use a program such as Free ISO Burner to burn the image file to a DVD. The image file should already be bootable so you shouldn't have to mess with boot settings.
After you create your installation disc, you can then install windows. Use the COA key on the sticker to activate windows. When the install is finished you may still have to go into the activation wizard and complete a phone activation. The instructions to activate can be found here.
After you have everything activated and it shows that it is a genuine copy, you can then go to install updates. These will take forever, but you need to update.
That is pretty much it. I do not condone using illegal Keygens or hacks to activate windows so please make sure you try a Microsoft source first and make sure that it's a copy that you have to activate. The key is what makes the copy of Windows legal or illegal. You always want to have a good, safe, and legal installation of Windows.
PS. I just tried the method with downloading the ISO from the second source and tested it, and it works. The only other thing to note is that if you are using Free ISO Burner, make sure to check finalize disc. See below.