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The company's stock price typically rebounds the next day, after investors realize that the slight decline on Patch Tuesday -- as Microsoft's regularly scheduled update release is known -- was unwarranted, said McAfee Inc.

But the fluctuations are so small that it would be tough for all but the best-funded investor to take advantage of the price movement, said Dave Marcus, McAfee's director of research. "Are the swings huge? Not really. But patches do have a trackable effect," Marcus said.

McAfee researcher Anthony Bettini, who authored a paper the company published yesterday, said that there was a correlation between Microsoft share-price fluctuations and the patch cycle. According to data Bettini tallied from 2006 and 2007, as well as thus far in 2008, there is a downward trend in the price of Microsoft's stock on the average Patch Tuesday.

On average, Microsoft's stock fell 0.11% on patch day during 2006, 0.29% during 2007 and 0.45% on those days thus far this year. Overall, however, Microsoft's shares climbed an average of 0.08% daily in 2006, went up 0.06% each day in 2007, and fell only 0.17% on average so far during 2008.

Even other Tuesdays are kinder to Microsoft's stock, said Bettini's paper: In 2006, Tuesdays without patch releases dropped an average of 0.01%, climbed by 0.15% during 2007 and rose 0.37% so far in 2008.

Prices typically rebound the day after Patch Tuesday, McAfee found. On Exploit Wednesday -- so dubbed because hackers have sometimes released attack code the day after Microsoft fixes bugs -- Microsoft's shares went up an average of 0.27% in 2006, 0.21% during 2007, and a whopping 0.49% this year.

"This is probably because institutional investors or market makers feel Microsoft was oversold the day before because of the bad news and that, in reality, Microsoft's value as an investment was only negligibly affected," Bettini wrote in his paper.

Just because Microsoft's share prices move down on Patch Tuesday then back up the next day doesn't mean there's a get-rich-quick scheme here, Bettini cautioned. "These price fluctuations are relatively small and tightly time-constrained," he said. "Profiting at a retail level from such trades would require risking a large amount of capital." He also noted that the data set was relatively small.

"Bear in mind, this is breaking a little new ground," added Marcus. "But it's clear that [Patch Tuesday] has some effect on [Microsoft share] value."

McAfee's research also proposed that the move to a regularly scheduled patch day -- something Microsoft did five years ago this month -- dampened the negative press the company was constantly getting when it issued fixes unexpectedly.