If you change hardware (such as a video card or hard drive) on a Windows XP computer many times, XP will force you to call Microsoft to register the OS again. It also unfairly forces users to buy 2 or more copies for each computer you own. The best thing is complain to the company and tell them you will not buy thier products anymore. Dont support companies that force product activation.
Product activation was designed to stop piracy. But the only thing it's succeeded in stopping is the installation of legitimate copies of Windows by users who rightfully own it. Activation often fails when someone upgrades the hardware in their computer (since the hardware identifier is different). People who pirate software can easily get around product activation by use of cracks and key generators.
I downloaded a cracked copy of Windows off of Warez and it doesn't even try to activate itself!