That comparison does not work. Microsoft pre-installing their applications, or tweaking their OS such that their applications provide a better experience is anti-competitive because it lets Microsoft utilize their monopoly to prevent competition.
Google is not preventing any other search engines from gaining popularity. People go to google to find the most relevant answer to a particular thing, when the top result is consistently a spam knol, people won't go there to search anymore.
The concern with Microsoft wasn't that they used their OS monopoly to unfairly compete with other operating systems, it was that they used their OS monopoly to unfairly compete with other applications (eg. IE vs. Netscape).
The concern with Google isn't that they might be using their search near-monopoly to unfairly compete with other search engines, it is that they might be using their search near-monopoly to unfairly compete with other websites (eg. Knol vs. Wikipedia).
Google is not preventing any other search engines from gaining popularity. People go to google to find the most relevant answer to a particular thing, when the top result is consistently a spam knol, people won't go there to search anymore.