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HOUSTON -- The Houston Texans have consecutive wins for the first time this season.

Never mind that they almost let this one slip away with a poor second half.

Steve Slaton scored two touchdowns and the Texans built a big lead before holding on for a 24-21 win over the San Francisco 49ers on Sunday.

The Texans (4-3) led 21-0 at halftime, then withstood a rally led by backup quarterback Alex Smith. Eugene Wilson's interception on fourth down halted the 49ers' last-gasp drive.

"We get down, they get up and they all of a sudden grabbed the momentum of the game," Houston coach Gary Kubiak said. "But we made some big plays late and defensively Eugene made a big play so it was a big, big win."

San Francisco (3-3) benched Shaun Hill after a terrible first half, and Smith threw three touchdowns to Vernon Davis to close the gap.

"I just thought it was time to make the switch," 49ers coach Mike Singletary said. "It was as simple as that. No long, drawn-out thought process. But let's make the change."

Singletary wouldn't say who would start next week.

Michael Crabtree started in his NFL debut and had five receptions for 56 yards. Singletary was pleased with Crabtree's performance.

"I was kind of calm," Crabtree said. "I practiced on getting calm and coming into the game, knowing what I was supposed to do and executing my plays. I think I did a good job, but I've got some more work to do."

Davis' third touchdown on a 23-yard reception got San Francisco within 24-21 with 3:49 remaining. Houston's only second-half points came on a 50-yard field goal by Kris Brown with about 7½ minutes left.

Crabtree's first game after ending a 71-day contract holdout Oct. 7 was overshadowed by the quarterback shake-up. In his first game since 2007, Smith sparked San Francisco's offense in the second half and was 15 of 22 for 206 yards. The former No. 1 overall draft pick made only one major blunder -- the desperation throw that Wilson picked off.

His first touchdown came on a 29-yard pass to Davis that made it 21-7, finishing a drive that included a 17-yard reception by Crabtree.

"Starting the second half, down three scores, you're seeing some stagnant looks from the defense, they're not throwing as much at you," Smith said.

Smith found Davis again on a 14-yard touchdown pass to cut the lead to 21-14 with about 13 minutes remaining. That score came 10 plays after Dashon Goldson forced and recovered a fumble by Slaton.

Mario Williams, who was the top overall pick the year after Smith, said dealing with the quarterback change was difficult.

"I didn't know who the guy was when he came out," Williams said. "I really didn't. It was one of those things where he definitely managed the game for them. He was getting rid of the ball on time and moving the ball."

Matt Schaub's 42-yard touchdown pass to Owen Daniels made it 21-0 in the second quarter. Slaton's scores came on a 1-yard run and a 9-yard reception.

Schaub was 20 of 30 for 264 yards and two touchdowns.

The 49ers couldn't get anything going in the first quarter, netting just 1 yard as Hill was constantly hurried by the Texans' defensive front. Williams, who's been slowed by a sore shoulder, got his first sack since Oct. 4 when he took down Hill on San Francisco's first play.

The 49ers said the play was ruined because of a miscommunication between Hill and Crabtree.

"More than anything, it was just lack of experience," Hill said. "But after that, he knew exactly what happened and he was ready to move forward."

The 49ers managed only 50 yards and three first downs before halftime. It was the fewest yards the Texans have allowed in the first half in franchise history.

Hill finished just 6 of 11 for 45 yards.

Game notes

The Texans were without Andre Johnson on their last drive after the star receiver's chest was bruised earlier in the fourth quarter. ... 49ers RT Tony Pashos has a left shoulder contusion. Singletary did not immediately know the extent of the injury. Also injured were LB Takeo Spikes (right shoulder strain), DT Justin Smith (left calf strain), NT Aubrayo Franklin (right knee strain) and LB Marques Harris (hamstring strain). ... Former presidents George W. Bush and George H.W. Bush attended the game.