For years, Microsoft was under the spotlight for antitrust, due to its market share and accusations of abusing its monopoly power. What started in the US in 1991 as a FTC inquiry came to a head with the DOJ case presided over by Judge Penfield Jackson, who ruled in 1999 that Microsoft indeed was a monopoly. The ultimate result was limits on certain contracting practices, disclosure of APIs and protocols, and rights for computer manufacturers to limit the visibility of certain Windows features in new PCs. The other "result" was the European Commission's ongoing follow-up of antitrust investigations of the company.
Now it may be Google's turn to go under the magnifying glass. While accusations of monopolistic practices have been heard for several years, in November the European Commission announced an antitrust investigation. On March 31, Microsoft announced it would join the antitrust complaint against Google in Europe.
Concerns have also been raised over Google's plans to expand Google Books and to purchase ITA, a flight data aggregation company.
Google currently owns about 66% of the search market in the US. However, they do more than just search. Google has moved into the mobile phone market with the Android operating system, which now has the largest market share for smart phones. They are working on a music retail service to compete with iTunes, have plans for YouTube to have a larger role in live sports broadcasting online, and want to create the largest digitized library of the world's books, searchable with Google Books.
While search can't lock in users, Android might. In addition, there are accusations that Google's massive search user base can affect a site's popularity.
As far as Microsoft's involvement - in 2009, Google joined in a complaint to the European Commission that Microsoft was shielding IE from competition. Microsoft may be happy to see the shoe on the other foot.