CAN’T ACCESS PCS ON A HOME NETWORK

I recently had an update from MS for service pack 2. I installed the updates and restarted the pc as instructed. I have 3 pcs wired into a router and until that point I had full access of shared files between them all. After the install of sp2 I was able to see the other pcs in my network places and even the shared folders. But when I attempt to access the shared folder from one on another. I get an error nessage telling me access is forbidden. I disabled the firewall and allowed for the sharing of folders on the pc running sp2 but now get asked for a password and user name. I have not installed ANY password and when i try to bypass the screen I still have no acces to the shared folders on the pc with sp2. I ran the network connections wizard and still no luck. please advise.