The best one will come form Microsoft of course and you could search their site for it. You can also quite legally use the rc.iso which forms the basis of all MS systems and will work on your system, be it 32 or 64 bit. You'll find that one here.
All that said, when it become illegal (Microsoft speak but better described as illegitimate) is when you add a system to eth ISO which is only the tool that makes a disk bootable. If you aren't using one that works in your computer and isn't in use elsewhere, you won't get way with it and if it is in use in another computer, that one may deactivate as well.
I am pretty sure I need a legal copy to upgrade from (have Win7 Upg install disks coming) and my legal copy of WinXP Pro SP3 suffered a fatality (disks disappeared during a move) when my primary drive decided it wanted to be a paperweight. I am installing 64-bit versions of Win7 Pro and Win8 Preview on a 2-drive system for programming environments. Really need the upgrade path.
I'm afraid to say the upgrade won't work. It's probably best to track down an end-of-life XP system - 32 or 64 bit will validate the upgrade. Try a charity shop for an old computer with an XP OS - that may be your cheapest option, having paid for the upgrade DVD.
Theoretically, any XP CD of the same system as your old one, will work to reinstall and Change Product Key to your own after it completes. You may need to talk through it with Microsoft on the Activation telephone line.
However, if the Product Key is on the case, and it was an OEN versoin, the above will not fly - you'd need an identical OEM version from an identical box because the setup will be tattooed to fit the driver set.
I wish I could say any of these answered my dilemma. Big box was a gift from a former family member with WinXP Pro, which I up'd to XP Pro SP3. Second box is an HP Pavilion from 2002 with WinXP Home--key on the side panel, up'd that to Pro. 40GB HD was imaged over to an 80 GB HD and that has moved back and forth between the two several times due to a desired improvement in the video setup, then failure of Big box MB.
There is little surprise that the hard drive gave it up (bad on me for not having an backup image). Just bad timing by a couple of weeks. Bottom line I have 2 (possibly 3) product keys.