Many security vendors are treating this day as if it were some kind of PC Armageddon.
"After today, there are no more security updates coming for Windows XP. EVER. And after today, NO ONE should be running Windows XP. ... make no mistake, you are now running a dangerous system and should get off of it as quickly as you can," security vendor Trend Microsoft wrote in a blog post.
The truth is, all XP PCs will continue to boot up and operate from this day forward, just like they always have. They will just do that without Microsoft's official support. That means that Microsoft will not fix bugs, add features, or - most importantly - patch newly found security holes.
Sort of. (More on that in a minute ...)
Because XP was the most popular operating system, it's also the biggest target for hackers, who have found thousands of bugs in it over the years, the most serious of which Microsoft fixed. Those bugs will stay fixed.
But security vendors warn that hackers will double-down on finding new bugs in XP.
Microsoft gave its customers plenty of warning. It told folks it would pull the plug on XP support today years ago.
The problem is that many, many companies are still using XP, for a variety of reasons. Lots of them are using software for their businesses that won't work on newer versions of Windows.
All told, almost one-third of PCs on the Internet, some 28%, are still using XP, according to OS watchdog site Net Applications.
And some 44% of businesses are still using it, according to mobile device management company Fiberlink.
That's why Microsoft is only sort of really pulling the plug.
For instance, the Dutch and British governments have both signed multimillion deals with Microsoft to get ongoing patches for XP for them, under Microsoft's Custom Support program, reports Ars Technica.
Banks using XP ATMs are reportedly negotiating a similar deal with Microsoft, reports Reuters.
Plus, Microsoft has agreed to continue warning everyone when it finds a virus on their XP PC, through July 14, 2015, it said. So if you use antivirus software, it will still work as a warning system, even though Microsoft is not promising to fix any new bugs that hackers find.
But the upshot is, if you're still using XP on an old home computer, be aware that it will soon be the target of hackers. If you are still using XP on your business computers, it really is time to get yourself to Windows 7 or Windows 8.