The workgroup names must be the same when sharing files between systems.
In Win 7, the default name used is Workgroup, while in XP...the default is MS Home.
I just went through a similar situation a couple of days ago
What I did was change the name of the workgroup on my two XP systems...from MS Home to Workgroup...and I've been able to easily access data files since.
The details for what I did are in the Win 7 Help & Support files, under Join or Create A Workgroup.
There are also instructions for doing so available on the Web, but I haven't tried any of those, since the Win 7 directions got me where I wanted to be.