Harang is 6-0 over his last seven starts against Houston, which has lost 10 straight games.
"A couple of years ago, I couldn't get a ball by them," said Harang, who tossed his first complete game of the season. "I remember that well."
In 2004-05, Harang started nine games against Houston and was 1-5. But the last two seasons, he's unbeaten against the Astros.
"I don't know what it is," Harang said. "I don't know if I'm throwing different or not.
"I just go out and try to be aggressive. It doesn't matter who I'm facing. I just try to throw strikes and make them put the ball in play."
Harang allowed two earned runs on six hits while striking out seven. He retired 14 straight after allowing a second-inning single to Adam Everett.
"Harang was outstanding," said Reds manager Jerry Narron, who was ejected in the first inning for arguing balls and strikes. "He has given us good outings almost every time out this year. With your No. 1 guy, that's what you're looking for. It was nice to see us play some good defense behind him too."
Harang (6-2) was getting his slider past the Astros all night.
"That's what [pitching coach] Dick Pole said, that my slider was real tight," Harang said. "I was able to throw it down in the zone.
"I went [to bench coach] Bucky [Dent] after the seventh and told him I could finish it. I figured I have an extra day off this week, so I could run the pitch count up a little [to 105]. And they were swinging early a lot."
Harang impressed his teammates.
"He did a phenomenal job," center fielder Norris Hopper said. "He was just doing his thing, and they couldn't get to him."
The Reds jumped to a 3-0 lead in the first inning, although they lost Narron in the process. Narron saw six pitches before being thrown out by home-plate umpire Bill Welke.
Hopper, who went 3-for-4, led off the game with a double and scored on Alex Gonzalez's single. An RBI double by Ken Griffey Jr. drove in Gonzalez. Griffey scored on Adam Dunn's single. It was Dunn's 500th career RBI.
Hopper has gone 6-for-8 since taking over the leadoff spot Tuesday from the injured Ryan Freel.
"He just puts the bat on the ball," Narron said of Hopper. "It's great to see somebody step in and do the job for us like that. I'm happy for the guy. He has put in a lot of time in the Minor Leagues to get here."
Said Hopper: "I feel real good, but it's only a couple of games. I've been waiting on this chance all year to show what I can do. Any time you get an opportunity, you have to give it your best shot. It's easy leading off when you have guys like Gonzalez and Griffey and Dunn behind you. I just have to get on base and let them knock me in."
The Reds have matched their longest winning streak of the year, having last won three straight during the first week of the season (April 5-7).
"That's the way we should be playing," Gonzalez said. "We know we are better than we've been playing. When you're struggling, you're struggling, and you just have to forget about it. But we've got a good team, and I think we're coming out of that [slump] now."
"You can just tell the atmosphere is way different in the locker room," Hopper said. "Everybody's working hard and working together, and that's all you can ask for."