Microsoft officials said the hackers did not knock its site off the web, but did manage to slow it down by about three to seven percent, reports CBS Affiliate KIRO-AM.
Meanwhile, National Discount Broker customers were unable to buy and sell stocks online. National Discount said NDB.com's site froze up early in the afternoon after the two companies that connect it to the Internet suffered technical problems.
Dennis Marino, the unit's chairman, said the two Internet Service Providers (ISPs) - Exodus Communications Inc. and PSINet Inc. - had yet to figure what went wrong.
Marino said the unit had had a similar problem with ISPs about a year ago.
Earlier this month, some other online brokers fell victim to computer hackers who jammed up their Web sites, but Marino said he did not know if that was the case this time round.
Marino said NDB.com was taking customer orders by phone until the problem was resolved.
He said the trading volume was "relatively steady over the past few weeks" and was no different earlier in the day.
One customer, Josh Landess, said he kept getting error messages as he tried to log on to the site.
"I couldn't get in at all," said Landess in San Diego, Calif., a self-described day trader.