I agree with the above post, also, if possible, stay clear of Vista, its not as stable as people would have you believe and is purely a practice run whilst they develop and finish "Windows 7". It was the same as "Windows ME", they released ME because XP was not ready for release in time for the year 2000.
Also, despite my recommending XP to you, I'd also mention that XP can only support 3MB of RAM whereas Vista will handle the 4MB of RAM that are appearing in most modern laptops.
So choice is yours.