It's the beginning of the end for the most popular Windows operating system in history.

As Hardware Central reports, Dell will no longer sell machines with Windows XP installed once September concludes, effectively putting a sell-by date on the nine-year-old operating system software.

The timing seems right considering Microsoft is about to celebrate the first anniversary of Windows 7 later this fall.

Microsoft isn't forcing PC makers to entirely pull the plug on XP sales, however.

"After the October 22 date, qualified customers will still be able to get systems with Windows XP through Dell's Custom Factory Integration (CFI) service...but only qualified end users meeting the custom image requirements will be able to get a new system with factory-preinstalled Windows XP Professional, XP Home, and XP Tablet products after October 22, 2010. This means that OEMs must stop the shipment of all XP orders on this date," the Dell blog post said.

Corporate purchasing managers have had plenty of warning that Microsoft would eventually stop letting PC makers continue to sell new machines with Windows XP installed.

Now, Dell says it will stop selling XP machines this month, a decision that must be a relief for Microsoft (NASDAQ: MSFT).

"Per Microsoft guidelines regarding Windows XP, system vendors like Dell will no longer be able to ship systems with XP Professional and XP Home after October 22, 2010 [the anniversary of Windows 7's consumer debut]. This means that we will stop offering XP as an option for customers starting this month in preparation for next month's deadline," said a post to the Direct2Dell blog Tuesday.

Of course, Microsoft officials have been saying the same thing for some time.