file sharing system help

i have a 500GB USB HDD that i want to use for file sharing and data backups.
my idea how i want to use it. is to have folders for each person and each person can access only their folder. but also have a shared folder where people can share their data. this is to be on a workgroup network. i already have something set-up for data backup. my family who uses the computers do not use much space. only i use a heck of a lot of space. i have a computer that i would not care what OS to load (either ubuntu or windows XP pro) if anyone can help me please do so. also if u have a question please state your qusestion too. thank you