2002–2009: Popstars: The Rivals and Girls Aloud
In 2002, Harding auditioned for the reality television show Popstars: The Rivals. The programme, the second British series of the international Popstars franchise, would see the creation of two rival groups – a boy band and a girl group each consisting of five members, which would then compete against each other for the 2002 Christmas Number One spot on the UK Singles Chart. Several thousand applicants attended auditions across the UK in hope of being selected. Ten girls and ten boys were chosen as finalists by judges Pete Waterman, Louis Walsh and Geri Halliwell. These finalists then took to the stage participating in weekly Saturday night live performances (alternating weekly between the girls and boys). Each week the contestant polling the fewest phone votes was eliminated, until the final line-ups of the groups emerged. Harding joined Nadine Coyle, Cheryl Tweedy, Nicola Roberts and Kimberley Walsh to comprise the new girl group Girls Aloud, formed through the show by a public vote on 30 November 2002.
The group's debut single "Sound of the Underground" peaked at number one on the UK Singles Chart, becoming the 2002 Christmas number one. Girls Aloud hold the record for the shortest time between formation and reaching number one. The group released their debut album Sound of the Underground in May 2003, which entered the charts at number two and was certified platinum by the British Phonographic Industry (BPI) later the same year. Their singles "I'll Stand by You", "Walk This Way" and "The Promise" have charted at number one. Two of their albums have reached the top of the UK Albums Chart: their greatest hits album The Sound of Girls Aloud and 2008's Out of Control, both of which entered the chart at number one, with over one million copies of the former being sold.
They also achieved seven certified albums and have been nominated for five Brit Awards, winning the 2009 Best Single for "The Promise".
The group's musical style was pop, but throughout their career, they experimented with electropop and dance-pop. Girls Aloud's collaborations with Brian Higgins and his songwriting and production team Xenomania earned the group critical acclaim, due to an innovative approach to mainstream pop music. The group became one of the few UK reality television acts to achieve continued success, amassing a fortune of £30 million by May 2010. Guinness World Records lists them as "Most Successful Reality TV Group" in the 2007 edition. They also hold the record for "Most Consecutive Top Ten Entries in the UK by a Female Group" in the 2008 edition and are credited again for "Most Successful Reality TV Group" in the 2011 edition. The group was also named the United Kingdom's biggest-selling girl group of the 21st century, with over 4.3 million singles sales and 4 million albums sold in the UK alone.
In 2006, Harding signed a £100,000 deal to model Ultimo lingerie, following the likes of Penny Lancaster, Rachel Hunter and Helena Christensen. Alongside her Girls Aloud bandmates, Harding made a cameo appearance in St. Trinian's in 2007. She took on her first major acting role in 2008, appearing in the low-budget thriller Bad Day. Harding wore a brunette wig for the role of Jade Jennings. The film also stars Donna Air and Claire Goose. She also collaborated with London synthpop band Filthy Dukes on a cover of "Real Wild Child", recorded for the film Wild Child. In July 2009, Girls Aloud announced they would take a year-long hiatus to pursue solo projects, but would reunite for a new studio album in 2010.
During Girls Aloud's hiatus, Harding decided to focus on her acting career. "Singing was always my main ambition, but acting was something else I wanted to achieve", Harding said. Harding appeared in the BBC television film Freefall, written and directed by Dominic Savage. The film aired on BBC Two on 14 July 2009. Harding portrayed Sam, the beautician girlfriend of Dominic Cooper's character. The drama is a satirical take on the 2008 financial crisis. Company magazine said that "Sarah Harding showed she's got real acting potential in her convincing, but fleeting performance."
Later that year, Harding returned to the St. Trinian's franchise with a starring role in St. Trinian's 2: The Legend of Fritton's Gold. Despite her age, Harding portrayed teenage rebel Roxy, a new student at St. Trinian's School. The film received overwhelmingly negative reviews, holding a 10% 'rotten' rating on Rotten Tomatoes. Harding worked with Girls Aloud producers Xenomania to record three solo tracks for the film's soundtrack – "Too Bad", "Make It Easy" and a cover of David Bowie's "Boys Keep Swinging".
