Go to Control Panel>System, scroll down to the Product ID to see if it has OEM in the character string. If so then you have the factory preinstalled Windows 7 still on there unless the seller installed a cracked illegal Windows 7 copy.
If you could see enough of the COA sticker to know which OS was licensed to the unit and provide some of it during phone activation after reinstall, we have reports MS will issue a new key. They have ways of knowing from the sticker if a worn Product Key was there.
If there is no OEM in the string then we assume it's a legal Product Key you can audit using Belarc Advisor
to Clean Reinstall - Factory OEM Windows 7.
If you'd like to stick with the current install you can also Clean Up Factory Bloatware
and Optimize Windows 7 - Windows 7 Forums
which may make it a keeper the way it is. A Windows 7 backup image would then serve as your Recovery.