IMHO use one of those two licenses; they're bought
and paid for and are legitimately yours.
AFAIK you've ALWAYS got the right to transfer the OS
license of an OEM installed OS if you transfer the hardware it is associated with, and clearly you're now the owner of that hardware, so the license is ALSO yours to do with as you please. It's not even debatable, it's all yours.
Although the "licensed device" of an OEM piece of software like those OSes is really tied to "that device", "that device" is a computer "system", not any PARTICULAR piece of that system. They couldn't legitimately deny you upgrading, repairing, replacing the PSU, the CPU, the RAM, the motherboard, the case, the hard disc, the keyboard, or any other component(s) of that system. And indeed if you had ordered repair service or upgrade parts straight from Dell they indeed WOULD gladly upgrade or replaced any/all of that stuff. There is no requirement that the serial number of any of that particular set of hardware remain the same (for indeed it'd change after the first repair / replacement or upgrade).
As you say, Microsoft's ONLY legitimate concern is that multiple PCs don't end up running the same OS, but so long as you own the system licenses / parts and are willing to designate that license's use on one given "system" (whatever mix of new / old / repaired / replaced / upgraded parts you configure that system with at any moment), it is only your business how you do that.
Now you may have a bit of a technical problem with respect to installing and activating the OS with that license. Some old OEM systems didn't include *any* means to *reinstall* the OS at all. Some systems kept a recovery image / backup of the OS on a "secret" hard disc partition, but the owners of the OS would be out of luck if their HDD crashed or was erased or upgraded though they'd legitimately own the OS licence they'd have no ability to install it! Some systems have an OS install / recovery OEM CD, but sometimes that CD isn't capable of doing a clean install at all, and installs all the old 20 year old cr*pware / trialware / spyware that was the default for that system along with the OS even though it's all obsolete and unwanted presently.
The worst part about those recovery CDs is that they often not only didn't offer a clean install option, but were also intolerant of operating on a system hardware configuration much any different than the default OEM hardware.
i.e. New BIOS version? Sorry, it won't boot / install the OS.
SATA disc? Sorry, there's no drivers for SATA, it won't work.
etc. etc.
So though it runs afoul of tons of anti-trust laws and consumer protection / warranty / first-sale laws, there can be a problem where you're basically denied the ongoing use of your OS license not because of legal restrictions, but because of technical brain-damage in terms of the OS installer not being useful / available.
They certainly would get sued into oblivion if they *explicitly* said: No, you have to have this model of INTEL CPU, you can't replace it with an AMD one. No, you have to have this model of seagate hard disc, not a maxtor. No, you have to use DELL 0.37 BIOS, you can't replace it with OpenBios or AMI if you want to and still be licensed for your OS. No, you can't reformat your HDD and install LINUX on one partition and reinstall Windows on the other without losing your Windows OS license....et. al. However IN EFFECT that sort of vendor lock-in / collusion / incompatibility is almost what they've been getting away with because of the difficulty in continuing to use the OEM OSes with limited or nonexistent reinstallation tools.
So they can't really DENY you your LEGAL RIGHT to use the OS properly transfered to you along with the original hardware set, you may have a nightmare trying to preserve / defend your right to do it in practice. If you have an OS install CD that will let you do a clean install or which has drivers / install support for your current hardware / software configuration you should be home free. If not, well, you could try to install off of another kind of OEM or Retail OS CD and try to reset the key to be the correct one for that system and activate it or not. Some people have been able to call in to the OEM's phone support / sales lines and order a "real" OS install CD for their license if they were deprived of a fully functional clean install CD with their purchase, or if that CD has been damaged / lost.
It is immaterial if the OEM (e.g. Dell) doesn't have YOUR name associated with that system serial number in their database. As long as you own it (didn't steal it), they have to ignore any details of its past ownership. If it is a very old system the warranty is probably expired so there's not even a question of warranty transferability. You're not claiming warranty service, only possibly the media / CD necessary to use the OS license(s) you legitimately own. If it makes the feel better they can add your name to the system's current registered owner database, they can't exactly refuse, or you can tell them to mind their own business, whatever.
As for Microsoft, they nominally don't keep track of personally identifying information with respect to WGA, though I think they probably do have a lot of "spyware" type of personally identifying information they collect even if they promise not to use it. In any case, it's irrelevant, if the system hasn't been stolen, the proof of OS license is that certificate of authenticity you have in your possession. If it was sold to you, given to you, whatever, it is not their business. Even if you lose the install CD, burn the hard disc, crack the motherboard and replace it with another, etc. you still own the OS license and they can't legally deny you use of it. The best they can do is make it an annoyance to go through the WGA / activation / phone activation, transfer to another department, please hold, did you check with Dell support?, .... crud before they solve your problem.
Frankly I'm amazed there haven't been big lawsuits about all the blatantly illegal things they try to assert are legal but which are in fact not. Check out the doctorine of first sale, the Magnusson warranty act law, et. al.
Good luck.
