network problem, please help.

hp computer with 10/100 net work card build in.windows xp installed, through a router conect to a workgroup. this computer can ping routers gateway, can ping 127.0.0.1. but can not ping its self. can not ping the same group computer. We are tring to connect to the other computer to use the shared document, but got trouble here. any body knows whats the problem?
we disabled all the firewall, uninstalled norton internet scurety,the computer can ping its self in the safe mode.
Thanks a lot for help.