Get in touch with your host and see if they can assist.
How do you know the system is under attack? What are you seeing?
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Indeed, DoS Deflate will not work on a Windows machine unfortunately, and assuming your attack is large you're going to want to address this at the network level primarily. The Windows firewall is not really designed to stop these sorts of attacks either. I believe 100tb may be able to offer Cisco Guard through SoftLayer - you want to contact them about this.
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