How to handle browser popup in Selenium using Java

Question

Generally, we scrap the data from websites using WebDriver with Java. During that scraping, if I want to download something (i.e XML file Download), there is a browser popup. Please help me know how to handle this browser popup using Java

<a href="javascript:downloadXML()">
    <img width="40" height="20" border="0" name="imag34" alt="Download" src="/pair/img/tabs/downloadxml.gif">
</a>

I am using the following code to click the image

driver.findElement(By.xpath("(//img[@alt='Download'])[3]")).click();

After clicking the image, it will open a "Save as.." popup. How do we go to that popup window and handle it?