Copy Files Across LAN

PC1 Win 7 Pro
PC2 Win XP Pro
PC3 Win 7 Pro
All in the same work group.

Setup PC1 to share a folder.
Not Simple to do. Dos anyone have a program to expedite this ?

But was able to see PC1 on PC2 in my network places.
So I found a subfolder there I wanted to use and mounted it as a drive
letter on PC1.

I copied 5 large files successfully. I had to log into PC1 from a
window on PC2. It was the usual login if at PC1.

I fiddled for an hour or two elsewhere and went to copy more files but
it was not allowed to copy or even look at the folders in my network
places from PC2.

I went to PC3 and easily copied files from there to PC1.
I think PC1 and PC3 are the same homegroup.

What happened to disallow working from PC2 to PC1 ?