As embarrassing as this is... I can't figure out how to install Windows XP without a CD. I have a valid CD key, but my sister lost her installation discs and now she needs me to reinstall for her, but I can't find anywhere to get ahold of Windows XP. I'm surprised I've never had this problem before. Does Microsoft really not give download access to their software to users who have a valid CD Key?
Since you have a valid key, and the key is all you're buying, just download a copy of the version she has (home/pro/oem/etc) and you'll be fine. As long as you install it using her valid key MSFT doesn't have the legal right to complain.