Yup, it's different with XP, you'll have to enable the Guest account in order to access the files on that particular XP machine. Just go into the Sharing tab of the drive or folder that you want to share on an XP machine. Then click the use simple file sharing, ignore the warning, after that, you'll be able to read and write files. Of course you'll have to do New Permissions since the C$ will be hidden to other computers.
If you have NTFS and later wanting to change the attribute of the files on that machine from another machine remotely, then just go into Folder Options, in the View tab, uncheck the box that says Use simple file sharing (Recommended)
That's all, hope this help.