On Thursday, software giant Microsoft Corp filed five civil lawsuits in Washington state court in Seattle, against the so-called "malvertisers," who allegedly transmit "malvertisements" online so as to hoodwink gullible users into downloading virus-ridden software on their computers.

Elaborating on the term "malvertising" in a blog post, Tim Cranton - associate general counsel at the Redmond, California-based Microsoft - said that it is a 'negative' type of online advertising, whereby the transmitters of these ads conceal malicious code under the guise of harmless online advertisements.

Cranton said the company is not aware of the people indulging in "malvertising" and expects the lawsuits to expose their identities.

The Microsoft filing, which cites unidentified defendants, has mentioned some business names - like Direct Ad Solutions; Soft Solutions Inc; Qiweroqw. com; ITmeter Inc; and ote2008. info - and has accused them of using Microsoft's system to post attractive online ads which entice users to specific sites that attempt to send across malicious software to their PCs.

Noting that "malvertisers" have not even spared mainstream Web sites, Microsoft alleges that, via their malicious ads, most defendants direct users to certain Web sites for a supposed security scan of their computers. Such sites generally warn users of security problems on their PCs, thereby asking them to install rogue security software, known as "scareware."