Ok, that rules out it being an iffy disk.
If the disk won't install on two different machines then it sounds like the disk is faulty. Which is probably why WH Smith sold it off cheap.
When a computer game is returned to some retailers instead of returning it to their suppler for a refund/replacement which they should do, they will first try selling it to somebody else in the hope the game will work for the new customer.
Obvioulsy can't prove that's true, but it's likely.
Try asking for a replacement from WH Smith but they'll probably just offer you your money back
Spore is available from Amazon for £11.39 btw.
And I recently picked up the expansion pack 'Galactic Adventures' from a GAME store for £7.00, they're selling lots of PC games of cheap atm.