Windows LAN Network is very slow

Hi,
I’m Using Windows 2003 Professional as a File server n i have 20 windows XP Pro Clients.n all connected to same workgroup.i connected this network with 2 D-Link 24 port 100/10mbps Unmanageable Switches. The problem is when i’m downloading some files from file server to other client or one client to other client the file transfer is very slow. To download 15MB file taking 5 Min n some times it’s happening with in seconds.