I recently bought a vista and downgraded to xp pro, probably did not have to anymore. I think windows 7 is coming out very soon
I think Win XP Pro is more stable and compatible with things on the market. I would recommend it for a trading system.
XP is much more stable and the main reason is that it was based on Windows NT platform which wasn't a MS product in the first place. (As far as I recall Microsoft bought server system from someone else and called it NT)
Vista laptop is fine! Main problem with Vista is on desktops where people want to have more than say, 4 monitors. A 4+ monitor setup in Vista gets tricky and more expensive than in XP. I recently bought 2 Dell systems with Vista/RAID-0 which I was very happy with. Until I got to the point of adding video cards. As a result, I've installed XP on 1 of those and do so on the other. I don't like Vista's video card options.
Will be getting wifey a notebook computer for her birthday soon. I still want to use XP, so Dell looks the most likely choice... Vostro 2510, probably.