Please see the following article for a utility to activate your windows. You need the Enterprise/Web key, not the Data Center key. (You get two when you have volume license. One is for Enterprise/Web, the other for Data Center)

Activate Windows Utility with GUI: http://www.symantec.com/docs/TECH182589

If you are using a proxy in your environment there are known issues with MS activation. Read the below if you want. The above will solve your issue. In addition you should update to the latest version to correct proxy related issues.

Configure for a proxy ( or Update to version 2.1 MP1 for Fixes to Proxy issues in v 2.1) :

http://www.symantec.com/business/support/index?page=content&id=TECH176835

Regards