Software giant Microsoft saw 11% wiped off its value on Friday after investors showed disappointment with third quarter earnings announced on Thursday.
PC users are waiting for the launch of Microsoft's Vista
The firm said net profits in the first three months of 2006 were $3bn (£1.7bn), a 16% rise on the same period a year ago, but below expectations.
Microsoft shares dropped by $3.10 to $24.16 - their biggest fall in five years.
That wiped $30bn off the stock market value of the company in a single day.
Money for shareholders
News that the company planned to spend more money taking on rivals like Google and Yahoo also contributed to the share fall.
"This is still a company that is extremely profitable," said Charles Di Bona, an analyst at Sanford C. Bernstein.
"What people are worried about is whether that ever flows through...to the benefit of shareholders, or does the company spend that money."
More than 50% of Microsoft's income comes from its Windows operating system and its Office business software suite.
But Vista - the first major update since Windows XP was introduced five years ago - will not be launched until 2007, instead of the second half of 2006 as planned.
Until then, Microsoft will be relying on sales from its Xbox games console and SQL database software to help bolster revenues.
The company is currently valued at $250bn (£137bn).