slow network performance

I don’t know what’s happening but the network performance suddenly drops down. The clients can download files from File Server only with 1Mbit/s on a 100Mbit/s network and upload files to the server with full speed of 100Mbit/s network. The server is brand new HP Proliant DL380 G5 running Windows 2003 Standard R2 x64 (with last firmware). Server is running as File Server, SQL and WSUS Server.

Transfer between other two older servers we also have is normal and the clients can normally use full speed of 100Mbit/s on both servers (Windows Server 2003 R2 x32).