OK unlike some problems all the machines can see each other.
This looks like Firewall / AV settings might be inhibiting access.
For testing disable AV software and switch off firewalls. If you are paranoia disconnect from Internet as well.
Now see if you can connect.
Another solution is to re-set all the share options on ALL the machines and then permit share on a USER basis (select the disk / folder you want to share, right mouse click, properties, sharing, advanced permissions, add -- then add your user id and the privileges you want).
On the Windows XP machines as well as the above re-run the Network setup wizard from the control panel. re-boot.
You should be able to access all the folders you want.
Now re-enable firewalls and...