What a difference half a year can make. In September, with a Yahoo (NSDQ: YHOO) purchase officially off the table, the Microsoft (NSDQ: MSFT) board authorized the company to spend $40 billion on buying back its own shares over the next five years. Microsoft CFO Chris Liddell said at the time, “These announcements illustrate our confidence in the long-term growth of the company and our commitment to returning capital to our shareholders.” So how much has Microsoft spent on buybacks over the last three months? Zilch.

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During the company’s earnings call Thursday, an analyst asked Liddell what was going on. “It is generally environmental. [I foreshadowed that] we would be generally more cautious in this current environment in terms of building our cash reserves ... We have built cash and investments back over $25 billion.” And, indeed, the company reported $25.34 billion in cash during the quarter, up from $20.715 billion during the previous quarter, when it spent $2.2 billion on buybacks.

Going ahead, Liddell said “we are still committed to repaying cash as much as possible, but it is going to vary quarter by quarter, and is going to depend enormously on the environment we find ourselves in, and things like our working capital management as well.” Translation: Don’t look for Microsoft to buy back stock this quarter either, considering executives said Thursday they did not expect the economy to pick up soon.

What’s not so clear though is why Microsoft has to be so miserly. Granted other companies are doing the same thing. But Liddell himself said that Microsoft is on track to generate cash flow of upwards of $20 billion this year even with the recession cutting into the profits of all of its business units. Microsoft’s stock also happens to be cheaper than it has been in almost 10 years. And its previous cash hoard of $20 billion was not measly, by any means.