Yeah I think the problem is XP eyefinity isn't supported on XP. This is on the eyefinity FAQ.
What Operating Systems does AMD Eyefinity technology work with?
At present Windows® Vista and Windows® 7 support AMD Eyefinity technology. There are plans for Linux support in an upcoming update to AMD Catalyst™ software. There is no support for Windows® XP.
its a limitation of XP its self that won't allow you to do it due to how many outputs it can support at one time from a single card. if i'm not mistaken it only supports 2 digital and 1 analog output(svideo) active per card.
it's time to upgrade the ol' OS.. trust me you will enjoy windows 7 way more than XP when you upgrade it, i absolutely can't stand XP anymore. i barely touch my old laptop which can't run windows 7 because of it.