Since I bought my OLPC, I have been a big fan of Sugar the XO OS. My kids have learned to use it and its simple interface with its vast amount of educational activities makes it a great OS.
While some have criticized the hardware of the OLPC, there is no reason why you cannot run this on any computer.
If you have a spare USB key (1GB or more), then you have NO EXCUSE. 
[5]
Visit the sugarlabs site at: http://wiki.sugarlabs.org/go/Downloads [6]
download the ISO [7],
[8]
follow the instructions for your respective OS 
[9]
boot from your USB key in any of your computers and be ready to be amazed!!!
Voila :)
Links:
[1] http://pulse.veltsos.com/category/it/hardware/olpc
[2] http://pulse.veltsos.com/category/it/open-source-foss
[3] http://pulse.veltsos.com/category/it/software/os
[4] http://pulse.veltsos.com/category/it/xo
[5] http://linux.philosweb.com/drupal/files/image_11.png
[6] http://wiki.sugarlabs.org/go/Downloads
[7] http://wiki.sugarlabs.org/go/Sugar_on_a_Stick/Strawberry#Download_locations
[8] http://linux.philosweb.com/drupal/files/image_13.png
[9] http://linux.philosweb.com/drupal/files/image_15.png