When it's a Linux distribution that looks an awful lot like XP. Not sure I would endorse this, if only for the fact that there is currently not a live CD version so you can try it on your hardware before installing - but it is "lightweight" and less susceptible to security issues that XP now is.
SoftpediaRight from the start you will notice the Windows XP resemblance, and the developers have made a great job imitating Microsoft's operating system. Someone less savvy in computers won't be able to really tell the difference.
The distribution comes with a number of applications already installed, like Konqueror, but users can also install Google Chrome, DebApps (manage and install applications from Debian repositories), Virtualbox Guest Additions optimized drivers for those who run inside VirtualBox, and Application Server / Client (accelerated application terminal server and client).
A very important package that users might consider installing is the Look Switcher, which will upgrade the way the distribution looks. It applies a different theme to the system and it will make it look a little bit more modern. It won't resemble Windows XP all that much, but you won't be able to shake that Windows feeling either.