Microsoft's business strategy has been intensely scrutinized for decades. So when the company last month bought a small maker of anti-spyware technology, the move was widely interpreted as a foreshadowing of its entry into the security software market. That prediction was borne out with the Jan. 6 debut of a Microsoft-branded spyware-fighting tool and the subsequent release last week of anti-virus software. While Microsoft has downplayed the significance of the new products, observers say it's a classic strategic move for the company, and is part of a looming overhaul of the two markets. How the giant company chooses to distribute the software will determine whether it again arouses the interest of the Department of Justice. Behind Microsoft's entrance into the thriving market for antivirus software is the rush of viruses that regularly rain down on PCs. In the month of December alone, 390 computer viruses were on the loose among the computing community, according to a tally by WildList Organization International, which collects reports from anti-virus experts throughout the world.