You buy Dell OEM software bundled with Dell's hardware.
OEM style software, at least from Microsoft is designed to be bought by system builders when they bundle it with hardware. That is you buy a bunch of computer parts from Newegg, you buy Windows OEM also. Then you build the computer and sell it to 'Bob' down the street.
Bob pays you, you pay Newegg, Newegg pays Microsoft. That's how it works, thats what is designed for.
(I realise that people often buy OEM copies of Windows as system builders when they build the computer themselves for themselves in order to save some money and it seems ok thing to do since Microsoft honors them when you do activation)
If your buying software for yourself your suppose to buy retail versions.
So if you want to buy Dell's OEM copy of windows, that's fine.. You just have to buy a Dell computer and they'd happly send you a copy of Windows you can install on that computer.
Of course it's completely worthless if you want to install it on a different computer. OEM licenses are tied directly to the hardware you bought the copy of Windows on.
If you want to install Windows on whatever computer you'd like you have to buy retail.
Now if you buy a Windows OEM disk and it's from Dell just from 'some guy on the internet' it may work. I doubt it..
But seriously it would just be easier to download XP off of the internet and find a key that works or something like that since that is about the same thing legally and it would be cheaper.
However if some guy is actually selling Dell-based OEM copies of Windows on the internet and he isn't representing dell.. then it's pretty likely that if you buy a copy and turn him in to Microsoft they may send you a free retail copy of windows as thanks for the tip off. Especially if it leads to a conviction.
So that's something you could do maybe.
Good luck on that one.