This excerpt taken from the MSFT 10-K filed Jul 30, 2009.
Impairment of Investment Securities
SFAS No. 115, Accounting for Certain Investments in Debt and Equity Securities, Staff Accounting Bulletin No. 111, and FSP FAS 115-2 and FAS 124-2, Recognition and Presentation of Other-Than-Temporary Impairments, provide guidance
on determining when an investment is other-than-temporarily impaired. Investments are reviewed quarterly for indicators of other-than-temporary impairment. This determination requires significant judgment. In making this judgment, we employ a systematic methodology quarterly that considers available quantitative and qualitative evidence in evaluating potential impairment of our investments. If the cost of an investment exceeds its fair value, we evaluate, among other factors, general market conditions, credit quality of debt instrument issuers, the duration and extent to which the fair value is less than cost, and for equity securities, our intent and ability to hold, or plans to sell, the investment. For fixed income securities, we also evaluate whether we have plans to sell the security or it is more likely than not that we will be required to sell the security before recovery. We also consider specific adverse conditions related to the financial health of and business outlook for the investee, including industry and sector performance, changes in technology, and operational and financing cash flow factors. Once a decline in fair value is determined to be other than temporary, an impairment charge is recorded to other income (expense) and a new cost basis in the investment is established. If market, industry, and/or investee conditions deteriorate, we may incur future impairments.