Read this full knowledgebase articles and know how do I activate Windows 2008 in easiest way.

All Windows systems automatically come pre-loaded with an activation key. Activation will occur automatically within 30 days of your VPS being provisioned, so there's no need to do it manually. If you do wish to do it manually, hit the 'Activate Now' button in the System Manager, it will prompt you to enter a new product key, leave this blank and hit 'Next'. This will then activate successfully.