Need an OS

[Nonmod]

I wouldn't saw this is Windows specific because it's not possible to run windows off a flash drive. Anyway, go ahead and try out SLAX (http://www.slax.org). If you're into pen testing and server security, try out BackTrack3 (www.remote-exploit.org/backtrack_download.html). both are based on Slackware linux but are easy to use. The cool thing about SLAX is that at the download page, you can get rid of what packages you don't want so you have exactly what you want. Hope this helped!

EDIT: Also, with SLAX, just click on add more modules in the "build SLAX" page. That way you can add exactly what you want and you have your own customized little SLAX system !

+1.SLAX is my choice for USB OS.

Also,you can try out http://www.susestudio.com ,with this building a custom SUSE is possible and gives you various hints on buidling os.