Why not install a user friendly free linux distro like mandriva, opensuse, fedora or k/ubuntu and set the free partition as the /home directory. You can limit their write access to that partition and if you get any hostile windows executables coming down the line, linux is not going to be abe to run them and windows will not be able to read from the linux partition.

PS-ubuntu is top of the linux charts at the moment, but the 'which distro' question is a different topic altogether.