[Skunkworks] Network Fails on windows XP

Hi Guys, I have this pc running on Win XP SP 2; The user uses outlook express and skype for most of the tasks. Network drives are accesible just fine. but skype will often fail to sigh in and if it does it wont be able to send messages, Outlook Express does the same. these two apps eventually work when you diable nd enable the network adapter/ connection or unplug and plug back the network cable... or restart the machine. The network is content filtered by DansGuardian running on a linux server, i checked with the ISP guys and the IP address on this machine is limited but allowed outlook and skype acess, there are several other machines on the network with the same limits but are working fine. Any ideas?? Maina Muraguri.