Sudden switch.

[funtoosh]

From thr day I have installed internet security software from norton sometimes the computers in my network would get local addresses starting from 192.168.x.x so they are not able to connect to internet. What could be the problem??

[niki]

Some firewall is preventing you to connect and this is problem with your router and not at your end.

[brodle]

All Norton is doing is confirming access to that IP range to see if you would like to put it in the trusted or internet zone, I would imagine if you have a router then 192.168.xxx.xxx might even be the addresses that the DHCP on your own router is offering you, Unless you are accidently connected to someone elses router.
Make sure you have not blocked your own Network addresses in Norton then get the command prompt up and type ipconfig /release until you see 0.0.0.0 then type ipconfig /renew and hopefully your internet will be restored, sometimes you might get on odd 168.xxx.xxx.xxx this is windows default and if you follow steps above will clear it, very common on a machine that has not been on your network before.