Loosing connectivity on router ports

Good Day All,

I have 4 computers connected to a 8 port d-link router(DIR-632). A Win XP(SP3) - running NIS 2011, Vista - running internet security from our ISP, another Vista -  runnning NIS 2011 and a Windows 7 - running NIS 2011. The computer running the  XP,  has file sharing turned on and the printers are shared throught this computer as well. It seems that all ports on the router lose connectivity/shutdown except the port that has the Win XP computer attached. I'm thinking that the NIS on the XP is blocking all the other ports on the router. If I re-boot the router, all the ports seem to get re-initialized and I connectivity will return to the ports. Is NIS causing this issue? All help will be much appreciated. Thanks to all in advance.
Zaboo