Thanks to all of you for your valuable suggestions.

Let me clear my network scenario.
1) My data sharing between Intranet is slow. Internet access is not so important as it has only 128 kbps leased line.
2)600 PCs are connected in four switches all over network. There is no IP address scheme followed for this network. i.e. We are using 192.168.X.X/class B & there is no IP segments in our network.
3)There is not a certain area where LAN becomes slow. Some time it becomes very slow or some times it is better.
4)Due to lack of segmentation of network, I can't able to locate exact faulty switch in such huge area.
5) I have tried escan antivirus on client PCs but couldn't find any virus.

Mean while I will check with Ad-aware SE.
Shall I convert existing network in four different IP segmetns, so that there will not be any broadcasting on other network untill any request comes.
Shall I use Layer 3 switch to coneect these four segments? or Linux PC as router will work ofr it?