Improve file transfer speed between Windows PCs and servers

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http://serverfault.com – I've setup a server which I've connected to multiple PCs in my workplace. Sadly, data transfer speeds are at max 3 MB/sec per connection which works out slow for file transfers, especially when transferring large files. I'm using Windows filesharing and the server is a Windows Server 2008 (2 Ghz CPU, 1 GB RAM) and the client PCs mostly running Windows 7. How can I detect bottlenecks in my network and improve file sharing speed within the network? (HowTos)