This campaign was a response to the highly successful Apple campaign that featured Justin Long and John Hodgman that (favorably) compared Macs to PCs. However, in a struck of irony, clever internet users figured out that the campaign had actually been created in Macs!
What they did is they looked at the 'metadata' or information that is embedded in the media files. When they looked into this metadata, they found that the files had been created on Macs using Adobe Creative Suite 3.
The explanation is quite simple; advertising is still a Mac dominated industry, and it makes sense that whatever agency Microsoft employed would use Macs. However, that did not stop Microsoft from scrubbing all metadata from those files completely.
Here's a sample of one of the ads