When you first turn on your computer, for about 1 to 3 seconds, at the bottom of the screen you may notice your computer giving you options. One of those options is a method of entering into restoring your computer. Like for example tapping on the F8 or F1 or F10 during restart. In restoring your computer, your should select at total restore. This will wipe the hard drive and will re-install windows assuming you have a secure copy on the D drive.
As a safeguard, you should make a copy of your hard drive before attempting a re-install, or make a recovery media which usually takes about 3 data dvd disk.
Every legal copy of Windows 7 comes with a COA sticker. Normally you would peal it off and stick it to the PC that you used that product code on. If you are missing the COA sticker then technically your Windows 7 is not legal. You have retrieved your product code so you could try re-installing that version using that product code and see if it activates OK. Who knows how long it will last though. If the seller tries to use that product code on another PC your PC could end up failing activation at some point down the road. Also as far as I know you can't sell that PC and advertise it as coming with Windows pre-installed without the COA sticker.
I would contact the seller and ask for the COA sticker and/or Product Key for the installed version of Win7. Tell him you are advised that selling a Windows OS without one is illegal - particularly if it turns out that it is pirated software - and you are wanting to clean reinstall so need the Product Key for the OS sold with it.
Other than that you can try reinstalling with the Product Key you audited to see if it will activate. Merely passing Genuine Windows test does not mean the OS is not pirated as cracktivations are done at the BIOS level to fool Windows Genuine safeguards.
If it will not activate after reinstall, buy an OEM version of Win7 to clean reinstall with it. Clean Install Windows 7
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