Hackers and spammers continue to exploit security holes in un-patched Windows operating systems.

This week Washington State Attorney General's Office filed a lawsuit against Messenger Solutions, a company in Scottsdale, Arizona, for sending out annoying pop-up ads "for pornography and Viagra" in order to coerce computer users to buy the company's pop-up blocking software.

The company used Windows Messenger Service (also known as Net Send messages) to deliver the pop-ups. Then it would send ads pitching its software that would block those pop-ups.

Windows Messenger Service, not to be confused with the instant-messaging program Windows Live Messenger, is primarily designed for use on a network and allows administrators to send notices to users. It comes pre-installed with some versions of Windows.

"Anyone who has Windows Messenger Service running on their computer enabled is open to receiving these advertisements," says Assistant Attorney General Katherine Tassi.

The lawsuit claims the initial pop-ups were just a setup to sell worthless and malicious software. The next set of pop-ups were made to look like security warnings that pitched a bogus software fix.

Messenger Solutions even offered a free trial version. If you clicked OK, the lawsuit alleges, malicious software was installed on your hard drive.

After the free trial period, if you decided not to buy the product, you would start getting bombarded again with more and more and more spam, essentially strong-arming you into buying the product.

But there's more. After your computer was infected, it was turned into essentially a spamming machine that would send out those annoying pop-ups to other computers. It would send out one of these pop-ups every two seconds, which would then start the whole scam cycle on someone else's computer.

The software sold by Messenger Solutions is called Messenger Blocker, WinAntiVirus Pro 2007, System Doctor, and WinAntiSpyware.

The AG's office says if you downloaded these programs -- even free trial versions --malicious software was put on your hard drive. That malware turns your computer into a spam machine that sends out the annoying pop-ups that started the whole thing.

The Attorney General's Consumer Protection High-Tech Unit says you may need a pro to remove that software.

This is not the first case where a spammer has exploited Net Send services. Service Pack 2 disables the feature on Windows XP. Vista users are not vulnerable. This is just one more reason to make sure your computer is set to get automatic updates.

Messenger Solutions could not be reached for comment.

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