Unable to browse network places in windows XP

Hi Everyone
i have a problem. we have a domain of round abouts 300 nodes. i have installed windows 2003 server on domain controller and windows xp on clients. on some clients now we can’t access network. whenever we try to access network plcaes it give message “unable to browse network”. We installed a fresh copy of Windows XP on some systems but after some time the same problem arise.
kindly if someone has the solution of this then please help me.
thanx
Jalil Khattak