Office Lan Network Problem

Hi,
please help me,

i surf the internet in the office through a proxy server set by our company at the same time i was able to see my office mate PC when through the windows network places. but just recently i got this problem, i still able to surf the internet but i can’t PING nor see my officemates PC through the windows network place. i even try to look their PC using the RUN in the start menu but NADA. i also try to ping all the Deafult Gateway, DNS Servers, and they all response. so why can’t i ping my officemates PC where they are just a few steps to me while the DNS Servers/Default Gateway are a building away? please help me..
thanks