Help please!!!!! Networking misery

I have 3 desktops ( and occasionally a laptop) all running XPHome (
different copies ) and all connected
via RJ45 wire to Linksys BEFSR41 ver2 4 port eitherfast/dsl router.
All can see the internet no problem, no firewall is activated on any (
during tests). Sometimes one shows the shared documents folder on one or two
of the other pcs but get the not authorised see administrator
messages when trying to connect.
I used to be able to share all folders/programs/drives between all pcs but
something seems to have changed and i cannot find out what it is. Network
wizard does not help other than insisting using a gateway/router that all
pcs have the firewall active ( this gets disabled during testing of the
network). I also used to be able to play network games between pcs but can't
do that either now.
At wits end.........can anyone help please???

Rgds
Kev