The past few days have shown the Redmond-based software giant is vulnerable to attacks by hackers and mistakes by its own technicians — a point acknowledged by the company Friday after hackers again caused problems for some people trying to access the company's Web sites.

The Redmond software maker said it experienced a so-called "denial-of-service" attack, similar to one that occurred Thursday and blocked access to sites such as Microsoft's main page, MSN.com and Hotmail.com — sites that serve millions of users.

"Through the painful lessons we've learned this week, we've already taken steps to change the architecture of our network infrastructure to improve its reliability and availability for customers," said Rick Devenuti, Microsoft vice president and chief information officer, in a statement.

The attack did not affect all cutomers, Microsoft spokesman Adam Sohn said. Many were still able to access the Web sites throughout the day and noticed no change in service, he said.

Web surfers were blocked from Microsoft Web sites starting Tuesday night. Microsoft restored the sites Wednesday after 22 hours, blaming it on a technician who reconfigured the network equipment that directs Internet traffic.

The next day, when sites were again inaccessible, the company said it was a victim of a so-called "denial-of-sevice" attack. Hackers flooded Microsoft's the equipment with fake traffic, thus blocking any legitimate Internet traffic, which meant consumers were again unable to access Microsoft sites.

The company maintained the initial outage was not related to the events of Thursday and Friday. It was trying to determine whether the Thursday and Friday attacks were related.

Eric Siegel, a consultant at Keynote Systems Inc., a company that monitors Web site performance, said his 120 monitors reached microsoft.com about 8 percent of the time and msn.com about 10 percent Friday, while normally the sites have 98 percent or greater success rates.

Some experts say it could have happened to any company, but the problems have been embarrassing for Microsoft, which is trying to encourage users to rely more heavily on the Internet as part of its .NET strategy, which turns its software into Net-based services.

"Microsoft is going to have to show a certain amount of robustness before people are willing to buy into this," said Ryan Russell, incident analyst with SecurityFocus.com.

Customers may be hesitant to trust Microsoft after several days of spotty Internet connection, said Gerald Altieri, network system engineer with Roseville Online, an Internet service provider in Roseville, Calif.

"It's going to put some more doubts in people's minds about having computers based solely on the Internet," he said.

With .NET, programs, word processing files and spreadsheets would be stored on larger computers that would be made accessible on the Internet, rather than on the hard drives of individual personal computers.

The software would be able to work with a variety of devices, including PCs, cellular phones and handheld computers. A programming language called XML (for extensible Markup Language) helps link users to the data they need.

Microsoft's new .NET strategy faces tough competition against rivals Sun Microsystems and Oracle Corp., which make large server systems, said Rob Enderle, an analyst at Giga Information Group.

"The large-scale systems are seen as more reliable. So when they break, people say it's an exception," he said. "(Microsoft's) smaller PC-based systems are viewed as unreliable. They break; they're still unreliable. It's more important for challenging technology to demonstrate they do not have these problems."

Microsoft lags behind Sun Microsystems and Oracle in large corporate business.

Microsoft spokesman Sohn said that Tuesday's error could not be blamed on the quality of the company's products, but a foul-up in the configuration of Microsoft's domain-name servers — the computer software that matches users with a particular address on the Internet.

It should not reflect on Microsoft .NET or the company's $200 million advertising campaign — launched Monday — that stresses the reliability of its enterprise software, Sohn said.

"We serve millions and millions of customers, virtually without fail," he said. "Unfortunately, we had an operational mistake with unintended consequences that lead to those sites appearing unreachable."