Ahead of the launch of its new operating system Windows 7, Microsoft chief executive Steve Ballmer has said that the company's reputation never recovered from the poor performance of Vista.

Ballmer said Microsoft's reputation took a beating after the release of Vista in 2007, an operating system which caused computers to hang indefinitely, among many other technical glitches.

"We got some uneven reception when [Vista] first launched in large part because we made some design decisions to improve security at the expense of compatibility. I don't think from a word-of-mouth perspective we ever recovered from that," Ballmer was qouted by the Daily Telegarph.

He said there is "pent up demand" for new equipment, tacitly accepting that Vista did not sell well.

"I'd be hopeful that we see a bit more rapid pick-up - particularly people who are upgrading the software without touching the hardware - than maybe we have in years past."

With Microsoft launching its latest operating system Windows 7 on October 22, Ballmer said the economic slowdown has hit sales of PCs which will in turn impact its sale.

Microsoft is likely to buy around 15 companies this year, in line with previous years, Ballmer told the newspaper.

Asked whether Microsoft would buy Twitter, he said the micro-blogging site does not "want to be bought".

"You'll continue to see us work hard and invest in the marketing and the like, and of course we're trying to get the Yahoo deal through regulatory," he said.