As part of the deal, Microsoft Digital Advertising Solutions will provide live video streams on an MSN of the launch event at 8pm tonight, during which the car will take a ride on the London Eye.
The video stream will also feature an interview with Andrew Humberstone, managing director of Fiat Group Automobiles UK, as well as a live announcement of the result of a competition to win one of the cars.
The MSN microsite has been up since New Year and has focused on "everyday masterpieces", featuring shorts about ordinary projects that we couldn't live without.
The microsite also links users to Fiat's own website where they can create a personalised Fiat 500.
Chris Ward, commercial director at Microsoft Digital Advertising Solutions, said: "Video is an exciting and innovative platform, allowing advertisers to push the creative boundaries online.
"The Fiat video shorts on everyday masterpieces and the live footage from the launch event are both great examples of ways in which digital content can be created to provide a unique and exciting experience for online users.
"It's great to see advertisers breaking the mould and creating bespoke material for their digital campaigns rather than just reusing content from other mediums."