network problem plz help me...

i am working in small company having 25 computers..

*one DSL router(internet connection)
*4 switches

i made a connection like this...


DSL router to switch(main)---->to switch1(connected 10 computers)----->to switch2(connected 8 computers)---->to switch3(connected 7 computers).

i have a problem when i am copying something from one system to another..
when i am copying just 2 or 3 mb file its taking about 10 15 min..in some systems it is ok but some systems having this type of problem

i thought it is a virus but how to remove virus from network
and kindly advice me what are the general trouble shooting steps to
solve this type of problems..
thanks in Advance

Whats going on is that part of the network works because it is on the same switch. But it all the switches are connected to the router, then it wont work unless you do some subneting. But when your router gets a packet, and doesn't know where to send it then the router will drop it. So you need to set up a gateway of last resort and forwarding domains.