can not connect to internet

dear friends,
i found new problem in winXP.i my office network we are using win proxy .
before 1 day ago my winxp works g8 and i am able to use internet too but
ones i start today it can't connect to internet even i m able to browse my
local network and also i m getting ping of my proxy server but i can't able
to get ping of DNS server. i don't know why this happens.

even i uninstallmy network card and again install it but same problem conti
..

have any solution or i have to install xp again ?

andy