Microsoft CEO Steve Ballmer has admitted that Windows Vista is an incomplete product, even though the operating system has been on the market for more than a year.

Speaking at the firm's annual Most Valuable Professionals event in Seattle yesterday, Big Steve told the gathered crowd that the unloved OS was "a work in progress". According to reports, he also promised that Microsoft would learn from the mistakes it has made with Vista.

"It's a very important piece of work. We did a lot of things right and have a lot of things we need to learn from," said Ballmer. "Certainly, you never want to let five years go between releases."

He acknowledged that "Vista is bigger than XP", but he wasn't referring to the popularity of the product. Instead, he was pinpointing one of the major issues many customers saddled with the product have complained about: performance.

"We have to make sure it doesn't get bigger still and that the performance and the battery and the compatibility we're driving on the things that we need to drive hard to improve."

Yesterday's admission will be seen by many as poorly timed, coming just weeks after service pack one (SP1) for Vista arrived – well, at least for some customers – in a manual form.

In mid-March Microsoft issued a staggering number of reasons as to why plenty of people would not be able to get their mitts on the service pack. Issues included a number of security products that won't start up or run on updated desktops thanks to "compatibility problems".

Earlier this week Microsoft spat out Vista SP1 in the remaining 31 languages. Those versions arrived a full month after the service pack first landed for a select few across the globe.

Meanwhile, the automatic version of the download remains missing in action. Redmond had chalked mid-April as the date when SP1 would start downloading onto computers across the world. But it's reluctantly stepped away from that deadline because it "wants to ensure customers have the best possible experience".

Ballmer also accepted yesterday that customers are incredibly reluctant to shake off XP in favour of adopting its unruly little brother, Vista.

"We have a lot of customers that are choosing to stay with Windows XP, and as long as those are both important options, we will be sensitive, and we will listen, and we will hear that.

"I got a piece of mail from a customer the other day that talked about not being able to get XP anymore, and we responded: XP is still available. And I know we're going to continue to get feedback from people on how long XP should be available. We've got some opinions on that."

Windows XP was given something of a reprieve earlier this month for bargain basement PCs not equipped to run the memory-chugging Vista OS. ®