I recently bought mine on ebay for £65 (full oem xp home version) No problems and was delivered quickly. The good thing about ebay is you can view comments from previous buyers re speed of delivery etc. I also paid by Nochex/debit card (see ebay) which seems a fairly safe way to pay. Don't know how the online retailers can sell so cheaply but it seems legal and above board.
I did the ebay route and got mine for £49 + P&P again check out the sellers feedback and previous sales, pay with paypal or nochex, mine came through 2 days after buying it sealed with the key I did the same with Norton system works pro £12 and Norton internet security 2004 £18
Try ebuyer click here In their search tag, put in 52362 for a copy of XP Pro with SP1 , or 50264 for a copy of XP Home with SP1. No conditions or other purchases necessary.Both full XP OEM discs, which can be activated via Microsoft.
Its not recommended,but this outfit are supposed to be able to get software dirt cheap.I was searching previous threads that mentioned them,only these threads appear to have been removed,and the link keeps giving "Page Unavailable..." messages,so IF it reappears,have a look and see what you think.