According to Ars Technica, the servers responsible for activating new installs of Windows and Office are currently down due to an unspecified reason. The issue became apparent after several users attempted to activate Office 2003 with no avail, even as early as this morning, and a Microsoft representative confirmed that the servers were offline. Fortunately, this seems to be simply a case of prolonged downtime, rather than an actual activation error, as was this case in August of last year, when the Windows Genuine Advantage (WGA) software acidentally labelled legitimate installs as pirated versions. Users affected by this issue can still take advantage of the built-in 30-day grace period until the servers are brought back up.
Update: We"re very sorry, but apparently our original story was inaccurate in more ways than one. A new email from Microsoft has revealed that only the offline activations were affected by the outage, which is to say, people who attempted to activate over the phone or through customer support, and that online activations remained unaffected. More updates as they come.