You should use their own Product Keys. If there are no COA's and they still had Windows on them then if they are Legal they have a Volume License Product in which case the Product Key & Licenses belong to the company who purchased the Licenses and unlike OEM Licenses are owned by the company who bought them and not tied to the Original Hardware that they are installed on.
If you use a Product Key from another company you are Committing Piracy not to mention Theft for the rightful owners of those Product Keys.
As for supplying anything unless you get supplied that Data you shouldn't be giving other peoples/companies property away and would be leaving yourself libel for the M$ Legal Department to pay you a visit and harass you for a couple of years before you agree to the Fine somewhere around the $250,000.00 Per unit and agree to have your Name & Contact Details normally your Home Contact Details published by M$ in an Enforcement Alert for those caught and pleading guilty of Piracy. These used to be Paper Documents that where sent to all M$ Partners but now I believe that M$ Lists Caught Offenders on the Web Site for all to see as well as sending out Enforcement Alerts to it's Partners.
For a single Domestic User they may get away with a simple act of Piracy but you had better believe that M$ Legal is very efficient in catching Large Scale Pirates and they are not particularly someone I would want Harassing me. I've had enough problems with them over the years and I do what they say needs doing.
I wouldn't want to be the but of their Action and the resulting issues that they raise. They are not people to mess with lightly or for any reason. Also if you are doing this for a company they will also be hit by M$ legal and get messed around quite a bit and end up handing you up as quickly as possible in an attempt to lower their Obligations under whatever M$ License they are/where using.
So once again if the systems don't have COA's on them and had Windows installed and they where Legit they had to be loaded with Volume License Product and if you are restoring them for the Volume License Companies Use you need to use their License Key/s and Volume License Product Install Media.
Volume License Keys [b]Do Not[/b] work with OEM or any other Microsoft Products so you really need a Volume License Install Media to load them
However if the units already have Windows Installed you can use a product like Magic Jellybean to get the Product Key/s that are on them and reuse those. Of course if they are not Legit For either the hardware or company you are committing an act of Piracy and are libel to the full effects of M$ Legal Department Actions.
http://www.magicaljellybean.com/keyfinder/
If these are unlicensed units I would either refuse to reload them or supply new M$ Licensed Software for each unit or offer the person/company involved the option of some form of Linux to be loaded. ;)
Of course if the person/company you are loading these for has bought them from their Original Owners who had a Volume License then they don't have any Legally Licensed Microsoft Software that comes with them and you have the choice of supplying OEM Product or loading a different OS and software.
I should also add that if you steal Product Keys from the place you work when M$ Legal Catches up they will proceed after the Company/Person who has the Computers, The Person/Company who Product Keys are used and the person/company who performed the act of Piracy. Over all no matter what else you had better expect a long troublesome few years and most likely Bankruptcy to escape the worst of M$ Legal. Personally I would prefer to walk into the Feds Head Office with a Fake Bomb and threaten them. Their response is far faster and much less painful.
You will of course have a Reputation as a Pirate that you deserve which will adversely impact on you in any future attempted employment or business. It's really not worth the hassles if you ask me. ;)
Col