You pick your Windows operating system the same way you'd pick it to install on a computer you just built. Bottom line, you want either XP or Vista OEM version - your preference. Whether you get Home, Professional, Media etc versions is entirely up to you based on your needs. The OEM versions are cheaper - don't come with documentation and their licensing is different, but are otherwise the same. One machine only. Buy the latest Service Pack you can - that just means that you won't have to download it. Note that current Service Pack for XP is SP3.