Is there a little star-looking icon in the system tray next to the clock?
That's the product activation "self destruct" application, and it is "legit."
I re-installed the OS on my computer after some hardware upgrades in a way that XP could preserve the existing applications and settings but change the core drivers. MS considers this along the lines of "ghosting" and prefers that you purchase a new OS license for the upgraded computer.
My computer refused to fully boot afterwards and came up with a product activation screen. I was unable to circumvent product activation unless I called the Microsoft 800 number provided.
I had a chat with a nice fellow from India about it. He asked why I installed a new motherboard on my computer. I informed him that my old one had failed, so I purchased a new one. He gave me a new activation code over the phone and I was back up and running after about a 10 minute phone call.
SWMBO had the little star-icon on her computer after some parts changes on her computer. She never did a re-install, but the star kept counting down. Once she hit 0 days remaining, it cut off her TCP/IP stack, making the Internet unavailable. She re-activated over the phone and was fine.
Just click on it, call the 800 number provided, and get it over with. Or switch to a different OS.