Anti-spyware maker Webroot Software has just confirmed what Windows users have long known: Spyware is a Windows-only phenomenon.

Recently, the company told the U.K. paper the Register that it hasn’t detected a single piece of spyware targeted at Macs or Linux machines. By way of contrast, the company’s Spy Sweeper software (which runs on my PC 24 hours a day), detects 15,000 pieces of spyware running on Windows machines.

The average PC is infected with 26 pieces of spyware, says Earthlink’s Spy Audit service, although that number is clearly inflated, because it includes certain types of cookies, as well as passive adware, in addition to spyware.

Why Windows? Internet Explorer and Windows itself has to take a good part of the blame, because of their inherent security holes.

But the operating system is only part of the problem. The bank thief Willie Sutton is reputed to have said when asked why he robbed banks, “Because that’s where the money is.”

For the same reason, spyware authors target Windows machines. That’s where the users are, and so that’s where the money is.

You don’t have to be a victim, though. Get the latest version of SP2, which includes a pop-up blocker, and helps stop drive-by downloads; install anti-spyware software like Ad-Aware, SpyBot or Spy Sweeper; use anti-virus software; and use a firewall like ZoneAlarm. (The Windows Firewall won’t do much good against spyware.)

So if you’re a Windows user, face the unfortunate fact: You live in a very crowded virtual neighborhood, and if you don’t lock your doors and Windows, you’ll be attacked.

Why do you think only Windows is targeted by spyware? Let me know.