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#21
Each purchase is a single licence to use on a single pc (at a time). Upgrade copies require an existing licenced copy of an older version of windows on that machine also.
If you subsequently buy a new PC, you can re-use the copy of windows 7 on this new pc; but only if you take it off the old one.
So in your case, to be legal, you need to buy a copy for your dell, and a copy for your hp. Since they both have windows already, you can buy the upgrade versions for both.
Yes, this means you'll end up with 4 discs (2 x64 and 2 x86) and 2 licence keys. The keys ARE usable on X86 OR X64; one key activates either version, though you're not allowed to use one key to install both discs on two different PCs; the 2nd activation will likely fail online activation.
2 upgrade retail copies = 4 discs = 2 licence keys = 2 licences = 2 pcs.