|OK... To clarify... as I understand it...: if you have a legitimate key (on the sticker on the PC case/housing) then provided the XP disk you use to install the OS afresh uses the same style of key; and is likely the same version - Home or Pro accordingly - you can use any disk to install the OS and that sticker-key to authenticate etc.?|
You will of course need ideally a full version - or an upgrade (providing you have a qualifying OS install disk to hand as well).
You can use an OEM version - as long as it's not one "fixed" for a specific branc of PC. eBay has OEM - non-fixed as it were - usually for not too many £££/$$$ etc.; and occasionally one can find none OEM full/upgrade disks too... But as you have a valid non OEM key already... I wouldn't seriously consider pursuing that option...; use a standard full/upgrade disk instead...
Also I don't think you're breaking any rules if you were to copy an XP installation disk - as long as "you have a legitimate key" - that "you" own and is effectively registered to/for you and your system...?
But as this was a Dell system originally... again perhaps chat up Dell and see what they may oblige...? Possibly they will offer/provide (for a few pennies...) a full XP disk?
Dell have been known to help out when a Dell installed drive goes down and the new/replacement drive obviously won't have that hidden/recovery partition...
You may also need to acquire (download) the drivers for the system as is (or was) - not the least motherboard chipset drivers - from Dell site.