The company that Microsoft has pursued is controversial: Claria, an adware marketer formerly called Gator, and best known for its pop-up ads and software that tracks people visiting Web sites. The Gator adware has frequently been denounced by privacy advocates for its intrusiveness.

The offer price on the table as recently as yesterday was $500 million, according to people who have been briefed on the talks. But a person close to Microsoft said last night that the negotiations were on the verge of breaking off.

One person briefed on the deal said there was opposition within Microsoft to the acquisition.

The anti-deal group, the person said, fears the move could bring an outcry as critics portray Microsoft as a corporate Big Brother, trying to track every mouse click on the Web and profit from it.