If you want a screenshot of your currently running browser instance then you can do it using following code:
public static void captureScreen(WebDriver driver, String screenshotFileName)
{
String screenshotsFile = screenshotsFolder+screenshotFileName+imageExtention;
try {
File screenShot = ((TakesScreenshot)driver).getScreenshotAs(OutputType.FILE);
FileUtils.copyFile(screenShot, new File(screenshotsFile));
} catch (IOException e) {
}
}
But if you want to take screenshot of your active window (other than browser only) then you can use Robot class.
Edit:
Although it's too late for you, the link below contains your answer. I think it may be helpful for others who are searching for the same thing.
Screen shot issue in selenium webdriver