“Guitar Hero” hit a sour note in its October launch as the videogame biz saw its formerly stratospheric growth tempered.New sales data from the NPD Group shows that “Guitar Hero: World Tour,” this year’s edition of Activision’s ultra-successful music videogame series, sold 534,000 units in its first week. While strong compared with most games, that’s less than half of what last year’s “Guitar Hero III” sold in the exact same time frame; 2007′s entry sold 1.4 million units in its first six days. Though “Guitar Hero” may have been hurt in part by MTV’s competitor “Rock Band,” which launched a sequel in September, its decline also reflects the slowing growth rates for the videogame biz. Overall industry revenue was up 18% in October, according to NPD. While still much stronger than other parts of the media sector, let alone the overall economy, it’s down substantially from last October’s 73% growth rate. October is the start of the holiday sales period, when vidgame publishers release most of their biggest titles and look for boffo sales. Not only was this monthly growth down substantially from last year, but it’s less than the industry’s year-to-date revenue growth of 25%, indicating that the holidays may slow down the biz instead of accelerating it. By comparison, last October’s 73% growth was a major boost to the then-year-to-date growth of 49%. One exception to the rule continues to be Nintendo, which saw its Wii sell a boffo 803,000 units, more than Sony’s PlayStation 3 and Microsoft’s Xbox 360 combined. Its handheld DS unit took the No. 2 hardware spot with 491,000. Thanks in part to a September price cut, the 360 easily outsold the PS3, with 371,000 units compared to a soft 190,000 for the PS3, which actually saw sales decline from September. That’s a worrying sign for Sony and also movie studios, since the device doubles as a Blu-ray player. Nintendo took several of the top software spots as well, with its “Wii Fit,” “Mario Kart” and “Wii Play” landing at Nos. 2, 4 and 5, respectively. Perf is particularly impressive given that the first two games came out in the spring and the last, which sells primarily because it comes with an extra controller, was released in 2007. Top game of the month was Microsoft’s Xbox 360 exclusive “Fable II,” which sold 790,000 units in its first month. Also debuting strongly was “Fallout 3.” Its Xbox 360 version alone sold 375,000 units, landing at No. 3 even though it was available for sale for just a week. Other new games with decent debuts include THQ’s “Saints Row 2,” which saw its 360 version launch with 270,000 units; Sony’s highly touted “LittleBigPlanet,” which sold 215,000 its first week; and the newest installment of its action franchise “SOCOM,” which sold 230,000. Electronic Arts’ new sci-fi horror game “Dead Space” had a relatively soft launch as its 360 version just cracked the top 10 by selling 193,000. Overall videogame biz revenue in October was $1.3 billion, up 18%. Year-to-date, it’s $13.1 billion, up 25%.