XP users can log into a Mac as an osx user, and read and write that user's files!

OSX: System Preferences > Sharing > Services > FTP access > Note IP address at window bottom

XP: Start > My Network Places > Add > Next > Next > enter Mac's ftp://IP > Next > Uncheck anonymous > Enter Mac's username > Next > enter name > Next > Finish > Enter Mac's password

This sharing of a Mac's files to a PC is the easier direction, because OSX comes with a great easy ftp daemon (turn it on with one click in the sharing system preference panel), OSX has a clean model of user permissions (just log in as a given osx user to access all their files) and XP comes with a great easy ftp client (a one-time setup of 10 clicks and several text entries give you easy access to an Explorer-like window allowing bidirectional file copy, move, deletion, and renaming). The opposite direction is tougher, because OSX lacks a graphical ftp client, and XP lacks an ftp server, requiring you to instead use msb, with its Windows-like insecurity, clutziness, and requirement of rebooting.