XP sp2 problem

Cant access Shared Folder on Windows XP Professional (SP2)

Just got this new PC. It can connect to Internet. It's OK to ping other PCs, but cannot ping itself. And other machine cannot ping this one. Under the "my network places" directory, cannot see the shared folders. Guess this may be due to Microsoft's firewall setting. Tried some time, could figure it out? Anyone has a solution? Thanks.