Its the fact that the security protocol of the newer systems, XP doesnt play well with for pass thru authentication. Easiest thing is to create a folder share on the XP system and use this for sending and receiving files between systems.
Other option is sort of dangerous and that is to set the folder permissions so that Everyone has access to it on the newest computer such as Windows 7 computer. This is safe as long as your router keeps out people from the outside, but anyone connecting to your home network would have access to this share location since its for everyone.
I generally create a mapped drive to a shared folder on a system that I want to share files with. This way for example if I have a XP and a Windows 7 system, the share is created on the XP system and on the Windows 7 system I have Drive Letter X: for the XP system. I can then drag and drop or copy/paste files to the X: drive and they are then transferred over the network to the XP systems shared folder that the Windows 7 system is mapped to.
