It was only last year that Britney Spears – once one of the biggest-selling pop artists in the world – was pictured shaving off all her hair and attacking a car with an umbrella. That was the year police were called to her house and she was admitted to hospital at Cedars Senai, after reportedly not sleeping for four days. It was also the year she lost custody of her two children. 2007 was the year Spears turned 26.

Tonight, however, is her big comeback – a whole episode of The X Factor based on her back-catalogue and featuring live performances from Spears herself. That it’s happening on UK TV rather than US TV tells you everything you need to know about the parlous state of her post-rehab career. In a nutshell, it doesn’t matter so much if she screws up here on a talent show full of karaoke wannabes. It’s not like screwing up on something proper, say David Letterman.

Because whatever Spears’ handlers would have her public believe, things might still go wrong. Spears doesn’t exactly qualify for a long-service medal in sanity and serenity. After all, it was only this time last year that she made her last big TV performance, at the MTV Awards – when it became clear that there was no one in charge of her career any more, least of all Britney Spears.

In that genuinely upsetting performance, one of the biggest pop stars America has ever produced wandered around the stage in ill-fitting fishnets and a hat, stumbling on the lyrics to her new single, Gimme More, and looking as though she thought it was all just a rehearsal – when, in fact, 10 million people were asking “WHO let this girl out in this state?”

However much you might believe in the miracles of rehab, it’s hard to believe that she would be ready, once again, to shoulder the huge burden of global promotion, world tours and media scrutiny that being a cutting-edge pop star with beautiful hair entails. Especially when that hair is just glued on while your own shorn locks grow back, slowly, underneath.

In many ways it was not surprising that Spears collapsed. If she is famous for anything, it is, perhaps, for being the greatest video star pop has ever produced. A state-level gymnast at the age of 9, star on the New Mickey Mouse Club at 11, Spears had a rigorous training in how to capture attention in three-minute bursts.

And she could. In her career-launching debut Baby One More Time, it was Spears’ idea to take her schoolgirl uniform and knot it up into a crop-top for the video. By the time of Toxic in 2003, Spears is frame-perfect. She’s an air hostess, a vampire, a biker girl. Her hair changes colour, she back-flips through lasers and blows up a building – but every time her eyes flick up to the camera, she looks like she wants even more.

If there is a main reason why Spears has sold 80 million records, it is this appearance of just wanting more. On last year’s Blackout album – recorded during her nervous breakdown – she was still unbowed. “I’m Mrs ‘Oh my God, that Britney’s shameless’/Do you want a piece of me?” she sang on the single Piece of Me.

In 2008, however, while she seemed more stable and sober, it appears to have come at the expense of, well, her guts. Her new album, The Circus, out next week, is a middling, dawdling offering.

“There’s no excitement,” she says in For The Record, broadcast on December 1. “Even when you go to jail, you know when you’re gonna come out. But in this situation, it’s never-ending. It’s just Groundhog Day every day. I never wanted to be one of those prisoner people. I wanted to be free.”

Her main preparation for tonight’s show has been, if the tabloids are correct, going to bed hungry every night to lose an apparently vital 7lb. As a pop fan, it’s hard to get excited about a performer who starves herself and finds her life desultory. Anyone who enjoyed her current single, Womanizer, might feel a wave of guilt – as if they’d just accidentally eaten some battery veal. Most people don’t want to watch someone who is performing because they can’t think of anything else to do, now that they’ve lost their children, given up drink and drugs and can’t go out for fear of being mobbed by paparazzi.

On tonight’s show, all Spears has to do is turn up thin and fairly sane, and the giant machine around her will conjure up enough magic to make it look like a victorious comeback. But coming back was never really the problem. It’s where Spears is going next that matters.

Rise and fall

— 1990s From the age of 11, Spears performed on a Disney Channel production with Christina Aguilera and Justin Timberlake

— 1998 Debut album . . .Baby One More Time reaches No 1 on the US Billboard Chart

— 2002 Much-maligned film debut follows the end of her four-year relationship with Timberlake

— 2003 Kisses Madonna on stage at the MTV Music Awards. Marries a childhood friend in Las Vegas – a union that lasts less than 3 days

— November 2006 Files for divorce from Kevin Federline, her second husband

— February 2007 Checks into a rehabilitation centre

— September 2007 Dropped by management company. Ordered to take regular drug and alcohol tests by the court dealing with her children’s custody

— January 2008 Attends hospital after a custody dispute. A court had ruled that Federline would have custody rights over their two children

Editor's choice

• U2 at Nou Camp Stadium, Barcelona
• The 50 Best Movie Villains
• The 15 golden rules of theatre etiquette