Rome - Microsoft chief executive Steve Ballmer on Tuesday said the computer giant had to reinvent itself to avoid being "old and tired" as his company struggles to keep up in the mobile devices sector."We're finding ourselves having to start up again," Ballmer said at a conference in Rome where he announced Italy had become the first country in which Microsoft phones were outselling iPhones."Unless you're constantly inventing something new, you're old and tired. Today we're having to remake ourselves," Ballmer told his audience.Referring to the success of Microsoft against rival Apple's iPhone in Italy, he quipped: "I don't know how long it's going to last".Ballmer was a classmate and friend of Bill Gates from their days at Harvard University in the 1970s.He took over from Gates in 2000 but earlier this year said he will step down by August 2014.When Ballmer took over, Microsoft was the undisputed tech sector leader, and the world's largest company in market value. But in recent years it has struggled as consumers began to move from desktop and laptop PCs to mobile devices.