As an addition to Microsoft's problem with WGA that we have been hearing about lately, there was an interesting bit of information regarding how accurate the program itself is in detecting pirated copies of Windows. Depending on the way it is worded, a statement from Microsoft has some believing that WGA has a false positive rate of up to twenty percent. This was taken from an article at Computerworld:

Through its spokeswoman, Microsoft said that “80% of all WGA validation failures are due to unauthorized use of leaked or stolen volume license keys.”
Unfortunately, no specific clarification has been received from Microsoft. The real answers probably won't come for a while. The article mentions Microsoft talking about product tampering and installations done by resellers incorrectly. In the complex scheme of Windows licensing, there are many things that could happen. Imagine someone who owns a Dell machine preinstalled with an OEM copy of Windows. Later, their installation breaks and they don't have their original restore CDs. They reinstall a retail copy of Windows and use a generated key. Though functionally the same, and while the person does own a license for Windows, WGA will flag them as a pirate. That 20% figure is very interesting, indeed.