This has cropped up every now and then, but currently the cheapest I think it's been. This is a retail version, not just OEM, so comes with software (though the site isn't particularly forthcoming about what that is)
EDIT 2: Price changed, now £43.48 inc. P&P, so back to being a good deal I reckon.
EDIT: Have expired as with P&P it's £49.47, so not quite so good. Still, better than ebuyer which only has the OEM for the same price.
now £39.98 with FREE delivery within 2-4 working days.