The woman accused of running over her adulterous husband with her Mercedes calmly described their disintegrating relationship to jurors Wednesday, saying she was moved to slap her "best friend" when she learned how far he had strayed.

Composed and often smiling as she took the stand to defend herself at her murder trial, Clara Harris said David Harris initially admitted only to being modestly intimate with his receptionist. His daughter told her, however, that the relationship was much deeper.

"I went to slap him in the face," Clara Harris said. She said her husband, a martial arts expert, fended her off, and she ended up on the bathroom floor.

The 10-year marriage ended a week later in the parking lot of a Houston hotel, when she ran him down after catching him with the other woman. Harris, 45, claims the death was accidental and that she shouldn't be charged with murder.

Harris described how the couple met as associates at a dental office, married, had twin sons and set up their own practices.

"We were best friends. We were very much in love," she said.

She said the two were "mature people" who had each survived divorce, adding: "We felt like there was no better couple than us."

But trouble popped up last spring as her husband was building his practice, she said.

"He was a little bit more retracted. He was a little bit more stressed. He was a little bit more intolerant of the boys," she said.

On July 16, an employee told her that her 44-year-old husband had been seeing receptionist Gail Bridges. Harris said that after she and her husband fought in the bathroom, he agreed to marital counseling, and he watched through a glass door as she fired Bridges.

"I asked her, `What kind of relationship do you have with my husband?' She said, `I don't understand,"' Harris said, relating the conversation using a mocking, high-pitched voice to imitate her husband's soft-spoken lover.

The Harrises spent the next two days discussing their relationship and how to fix it, she said. At an airport bar, Harris said she took notes on a napkin, making a side-by-side comparison of her and her rival. Where David Harris said his wife was pretty, smart and educated, he said his lover was "reasonably" pretty, "reasonably" smart and "reasonably" educated.

She testified her husband listed Bridges' advantages as communication and willingness to allow him to do "anything he wanted to do."

Harris testified for about 1 hours in the morning. Afternoon testimony was canceled after her attorney George Parnham fell ill. The trial was to resume Friday.

Parnham had not yet questioned his client about the evening of David Harris' death. Shortly after the lunch break, Parnham grew woozy and buckled in a courthouse hallway.

Wendell Odom, one of Parnham's partners, attributed the possible fainting spell to a combination of flu and stress.

"George is all right and the trial will continue as soon as we get him back," Odom said.