Access Is Denied To One Computer, But Not The Other

I have 3 computers on my network connected to my router. My main desktop
which has Vista 5384.4 on it is connected via ethernet. My other desktop
with Windows XP SP2 is connected via wireless as is my notebook (Windows XP
SP2). I can move, rename files etc on my vista computer from my notebook as
I have always been able to do. However, I can only read files on my vista
computer from my other desktop. If I try to move or rename files on my vista
computer from my other desktop I get the "Access Is Denied" message. I need
to be able to modify ALL my files on ALL my computers FROM ALL my computers.
Can anyone help me please? I don't know what I need to do to my second
desktop to make it behave like my notebook.