Whatever you choose, perhaps better not start with Vista anymore.
MS will only support it until April 10, 2012, so that's only about 6 months left.
However: The extended Version (for commercial customers of MS) will be supported 5 years longer.
Support means in this case: automatic fixes, updates or online technical assistance.
Here in short the 'lifespan' of some versions, perhaps it will help you decide:
MS will support XP until April 8th, 2014.. the support for younger MS Vista version will already stop April 10, 2012.. WIN 7 will be supported until January 13,2015 (the business editions Windows 7 Pro/Enterprise) will get extended support until January 14,2020