[Santiago]

XP Pro would be my choice.

New copies of XP will stop being sold in 2008 but still supported til 2012.

Vista is nice and has some improvements but has hi system requirements is is full of conveniences but not essentials.

Vista will take at least a year to settle down. If you're happy with trying to keep up with all the driver updates especially video drivers, then go for it. Note: Nvidia drivers are loaded with bugs and there are several class action lawsuits being formed. Nvidia's "Vista Ready" just isn't. But about half seem to have some kind of problems and not just with games. Hardware too.

Microsoft did such a horrible job of working with vendors on legacy XP-Windows 2000 application-hardware compatibility that everything is up in the air. Don't expect windows vista drivers for hardware you already own until late 2007 at best.