Congratulations, Microsoft (MSFT) . You’ve finally joined the world of online advertising.

The company’s money-losing MSN online division has been stuck in third place in Internet search for some time. And it was starting to look like it was going to remain an also-ran for the foreseeable future.

Microsoft lost out to Google (GOOG) in the bidding war for DoubleClick. Madison Avenue ad agency (WPPGY) wound up scooping up 24/7 Real Media (TFSM), another company that Microsoft was said to be pursuing. And those merger talks with Yahoo (YHOO) don’t appear to be going anywhere.

But fortunately for Mister Softee, all it had to do was venture out in its own backyard to find a company that instantly catapults it into a leadership position in the online ad business. Microsoft’s $6 billion offer for Seattle-based aQuantive (AQNT) is a brilliant move for the company.

aQuantive operates in three different aspects of the online advertising world. Its digital marketing services business includes Avenue A/Razorfish, an online ad agency. It competes closely against Digitas, which was recently purchased by traditional ad agency Publicis.

aQuantive also owns a digital marketing tech division. The company’s Atlas product serves and places ads. So this business competes mainly against DoubleClick. Finally, aQuantive’s digital performance media business owns DrivePM, an online ad network that competes with the likes of 24/7 and ValueClick, the last remaining publcly traded online ad firm that hasn’t been bought by a larger company.

So in one fell swoop, Microsoft is bulking up in several key aspects of the online ad business. It probably won’t need to make any more significant purchases. If Microsoft had been successful in its efforts to acquire DoubleClick or 24/7, the company may have still had some holes to fill.

This could be one reason why Microsoft is deciding to pay such a gargantuan 85 percent premium for aQuantive. On the surface, the $6 billion price tag seems more than a tad frothy. But this is Microsoft after all: $6 billion accounts for only about 2 percent of the company’s $293 billion market value. Even its cash on hand is more than $28 billion.

“It’s expensive but once you get an environment where people are bidding up assets, you definitely will have to pay a premium for things. In the grand scheme of things, it’s not that big a deal. What will be more important is what Microsoft does with those assets. But this is a further commitment to strengthen MSN,” said Richard Williams, an analyst with Summit Analytic Partners, an independent research firm focusing on the software business.

What’s more, Microsoft is paying for the purchase in cash and not stock. So MSFT owners don’t have to worry about more shares flooding the market, which could hurt earnings per share. To that end, Microsoft chief financial officer Christopher Liddell told analysts during a confernce call Friday morning that the company felt no need to change its financial guidance for fiscal 2008, which is when the deal is expected to close.

Liddell boasted about how there were only a few companies in the online ad world that would give Microsoft “critical mass.” And certainly, Microsoft needed critical mass to be considered a more serious player in online advertising.

So all of a sudden, the world of search and display advertising just got a lot more interesting. Microsoft, which I sarcastically dubbed the Dr Pepper of search not that long ago, finally has made a move that could enable it to move out of a distant third to Google’s Coke and Yahoo’s Pepsi. Folks at the Googleplex and in Yahoo’s Sunnyvale, Calif. offices now have 6 billion more reasons to fear those operating system guys out of the Pacific Northwest.