I'd recommend a Debian derived distribution because those are most similar to Maemo regarding package management and such.

Just download a live CD or live DVD of Ubuntu or Kubuntu (I recommend trying both) and get started. You won't even need to swap HDD or anything, it will run right off the CD or DVD. But that has to be loaded in RAM first, so you need to be patient. You could also try Wubi. It installs Ubuntu on your XP partition without breaking the XP partition.