XP/W7 networking problem

I have three machines networked

2 Windows 7 (1 Pro 64 bit/1 Ultimate 32 bit)
1 Windows XP Pro 32 bit

XP machine has TWO logins, the regular user my wife does NOT have admin
privileges, as her teenage daughter occasionally uses it, and I don't want
her downloading whatever! I have a login, with admin privileges, for
maintaining the machine.

Same workgroup; all set up correctly according to the MS video on networking
machines with a mix of 7, Vista & or XP

I have a large music library on the two W7 machines, with homegroup access;
they are also specifically shared, so that they can be accessed on the XP
machine.

All set up yesterday, and was running fine; after the setup, I stopped all
three machines, powered off and on the cable modem and router, and then
rebooted the machine. Access to all the files correct on all three machines.

Today, the XP machine can only see ONE of the Windows 7 machines. Am stuck
as to what is going on. The machine has an automatic backup at night, and
then shuts down. The only difference today is that my partner logged in
first, though I can't see why that should make any difference, especially as
ONE of the W7 machine IS visible.

Please advise! It's driving me nuts.

I do plan to migrate the XP machine to W7, but as my partner has all her
work details, in Outlook and Excel, on that machine, some care needs to be
taken, so I want a day on my own to handle that.

Am very experience with Windows installations, from 95 onwards, and have
been working with Windows PC since the glory glory days of 3.1! But this one
has got me stuck

My Network Places on the XP machine will NOT let me access the workgroups. I
had mapped the network drives so that they can be seen in My Computer. The
shares on the "lost" machine are visible, but you get an error when trying to
access them (sorry, message not to hand, but it is a standard one for a lost
network connection) and trying to access the workgroup I am told I do not
have sufficient rights.

I - as admin - AM logged on to the the machine, but AFTER my - non-admin -
partner. Could it just be that I have to log on first?

No password needed to access the shared files; the XP machine set up as it
were a Homegroup machine, with Read/Write privileges, and as noted, all was
fine yesterday.

Grrrr....