This excerpt taken from the HPQ 8-K filed Nov 12, 2008.
Marketable securities consist of government and agency obligations, corporate debt and corporate equity securities. The Company classifies all of its debt and marketable equity securities as trading or available-for-sale. All such investments are recorded at fair value. Changes in net unrealized holding gains (losses) on trading securities are recognized in income, whereas changes in net unrealized holding gains (losses) on available-for-sale securities are reported as a component of accumulated other comprehensive loss, net of tax, in shareholders equity until realized.
Investments in marketable securities are monitored for impairment and written down to fair value with a charge to earnings if a decline in fair value is judged to be other than temporary. The Company considers several factors to determine whether a decline in the fair value of an equity security is other than temporary, including the length of time and the extent to which the fair value has been less than carrying value, the financial condition of the investee, and the intent and ability of the Company to retain the investment for a period of time sufficient to allow a recovery in value.