Can't Access The Folders On The Vista

I have 2 XP machines and one new Vista machine. I have set up my wireless/wired network just fine. My 2 XP machines can share files and I can use either computer to access the other's files. Enter the Vista machine. I set up the network as requied and have a Workgroup. I can "see" the names of the files/folders being shared on all machines. I can use my Vista machine and move files back and forth, access them, etc. But when I sit at either of my XP machines, I can't access the folders on the Vista machine. When I try, I'm told "access denied." I've eliminated my firewalls, I have allowed full sharing. But nothing works.