On windows 7 avoid
enabling or joining a Homegroup and networking with XP machines will be a little easier.
Not meant as a How-to, just something to keep in mind next time this diseased Redmond-borne
option comes up to leave it alone and not enable it.
When sharing folders you have to "allow network users to change your files" and/or you might have to create the same users with the same password (an actual real password- not blank) on the machine that is sharing the folder.
And sometimes you have to jump over the humps that internet security software puts in front of you.
It kinda depends on what kind of "mess" each machine is in
Networking w7 and XP is completely doable though.