A high-level marketing executive at software giant Microsoft Corp. resigned his post suddenly this week with no explanation, leaving a cloud of mystery and speculation. The executive, Martin Taylor, was a 13-year Microsoft veteran who most recently served as corporate vice president of marketing for Windows Live and MSN. "We have made the difficult decision to part ways with Martin, but we don't comment on personnel matters," the company said in a statement. "We appreciate Martin's contributions at Microsoft over the past 13 years." Taylor is credited with helping Microsoft beat back the threat posed by the free Linux operating system when he was head of platform strategy with the Windows group. His exit leaves Microsoft searching for a new marketing expert to anchor the company's efforts to win customers from Google Inc. "This is a problem because this is supposed to be the big marketing thrust for Windows Live," said Paul DeGroot, an analyst at Kirkland-based market research firm Directions on Microsoft. "It's important that they get the brand out, and that they have somebody in that position that has the confidence of senior management and can get things done."