cannot open shared folder

I have two PCs in my home connected via a wireless
network. I have setup a shared folder on the primary PC
to transfer files to and from the secondary PC. The
system worked fine last spring. But now several months
later I cannot open the shared folders while at the
primary PC. If I turn on the secondary PC I can open the
folders and open files in the folders. On the primary PC
if I try to open a shared folder it displays temporarily
(<2s) then closes the explore window and all other open
windows. Sometimes the 'Send Microsoft an Error Report'
thingy comes up - but not always.

I am operating Windows XP V5.1 SP1. All other
functionality on both PCs is fine.

I'm not a Windows or IT SME - so if you reply make your
instructions clear and concise (for dummys).

Thanx in advance for your help.