Einhorn's fund, Greenlight Capital, owned more than 9.9 million shares in Microsoft on June 30, according to SEC filings. That was good for a 0.11 percent stake in the company, according to Bloomberg data, making Greenlight one of the top 100 owners of the company.

Here's some of what Einhorn wrote, explaining the decision to sell off his Microsoft position, too late, "as the shares peaked after [Microsoft] announced a very good September 2007 quarter." From SAI:

"Since then, management has acted in an overaggressive and almost panicky fashion regarding its online offering. First, it sought to acquire Yahoo! and then after that failed, it announced extremely high internal investment requirements to pursue this "huge" opportunity (read: "Google-envy"). We doubt the opportunity is what they say it is and wish MSFT focused on its core strength: software.

"The CEO is a very smart and very wealthy man. Perhaps, he is so wealthy that he has bigger ideas and aspirations than making MSFT’s shareholders wealthier. We've given up on MSFT for now as we feel better investing in companies where management at least appears to be trying to work for shareholders."

For the record, the CEO, Steve Ballmer, is the company's second-largest shareholder (behind co-founder and Chairman Bill Gates) with 408.3 million shares, or about 4.5 percent of the company. The $3.09 that Microsoft shares have lost this week works out to about $1.26 billion shaved off of Ballmer's holdings.

Is anyone else evaluating their portfolio and making a change in their Microsoft position? Tell us why you're buying, selling or holding.