Microsoft has rolled out its self-serve advertising program adCenter across all its US search queries, a year after the program was first announced.

The move, which now pits it against Google and Yahoo in the ad serving business, should help the company increase advertising revenue.

Like Google adwords and Yahoo's Overture services would be advertisers will bid on keywords to be displayed next to search queries displayed on the page.

Microsoft claims that most of the advertisers who have participated in the beta have seen better conversion rates that Google and Yahoo, while the company boasts that MSN reaches nearly 2 out of 3 web users and a whopping 40 million people a month.

Microsoft is keen to get a piece of the action. Last year companies spent $5.1 billion on search-related ads in the US alone, up 31% from the year before, according to the Interactive Advertising Bureau trade group and PricewaterhouseCoopers LLP.

Microsoft is expected to start testing its AdCenter service in the UK next month, followed by other countries thereafter.