Same internet connection is fast on Windows 7 Pro, slow on Windows 2000 server

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http://serverfault.com – in my company we have an old Dell running Windows 2000 server and ISA Server and it's our Internet gateway / firewall. Our current Internet connection is an ADSL and it's fairly slow (about 1 Mb/s in download, even slower in upload). But now we have bought a cable connection from another provider, and we signed for a nominal 10 Mb/s. Before switching to the new Internet connection I tested it for a while using a Windows 7 laptop, directly connected to the Ethernet cable provided by the ISP. It worked fine, and http://speedtest.net was giving me 8.5 Mb/s in download and around 5.5 in upload. (HowTos)