This testing version – or first release candidate (RC1) – is the version of the operating system that will be put through its paces by businesses before the official launch of Windows XP on Oct 25, Microsoft said in a statement. So far, over 500,000 beta testers have been using the highly anticipated Windows XP, Microsoft said. A second release candidate version is expected in July or August. One of the features included in Windows XP RC1 is Windows Messenger, Microsoft's IM (instant messaging) service which will attempt to take some market share away from dominant player AOL Time Warner and its AIM (AOL Instant Messaging) service. According to Microsoft, over 100,000 participants have signed up for the Windows XP Preview Program, which – for a fee – allows users to test versions of Windows XP.