In every case I see it's due to sloppy repair of a failed disk or system when the owner has had someone reinstall Windows and the repair installer has used an invalid or illegal Product Key or Volume Licensing Key instead of the owner's perfectly valid copy. I cannot begin to guess why this is done, since it simply isn't necessary and ends up costing the end-user in the long run.
How painful is it to pay somebody to come and fix the problems caused by the guy you originally paid to come and fix your computer problems?
Recently I came across an instance of this, for which the re-installer had used a different version of Windows XP, that is Windows Professional, instead of Windows XP Home. Sadly that meant that instead of just reinstalling the valid XP Home Product Key, which would not work for the XP Pro installed on his system, the owner had the choice of re-installing Windows from scratch, or buying a new license from Microsoft for $149.
Neither choice is painless, but in this case, it was less painless for the owner to simply buy the new license to make his system legal and avoid being harangued by Microsoft over his illegal system.
And that's what will happen. When Windows is updated it installs a validation program as well as the updates to see if the operating system has a valid Product Key and Activation. If this program, the so-called, WGA, or Windows Genuine Advantage program, fails to find a valid copy of Windows, it begins to complain all the time, on login, and randomly interrupts the user at work as well.
The same thing happens with Microsoft Office -- I regularly run across systems with illegal Enterprise Microsoft Office licenses installed which require either a removal and reinstall with a valid license or the purchase of an Enterprise license from Microsoft -- whoa that's EXPENSIVE.
The moral of this story is: Absolutely and positively make sure that your support person uses your legal and valid software and version when he reinstalls it, and that he enters the correct Product Key for the license you own. Period.
Every computer system you buy running Windows, from XP through Vista, and to Windows 7 comes with a sticker on the computer with your Product Key and Windows version listed. Save that information. Write it down. Paste a copy on the reinstall or recovery disk.
Also most computers still come with a reinstall disk from the manufacturer, such as Dell, or HP, or whomever. Guard that disk well. I keep mine either velcroed to the side of the machine or in a pouch taped to the side of the machine. I do not want to misplace it.
Some systems now do not ship with reinstall software, but only come with a partition of the hard disk with so-called "recovery" software installed on it, which will return the system to its as-purchased state. Unfortunately it cannot recover your disk if the disk itself has failed. If your computer does not automatically come with a reinstall software or a recovery disk, do buy it as an option. It's usually less than $20, and will save your bacon if you need it.
OK, one last thing. All this talk of re-installing Windows software compels to me to add:
If you have to reinstall your OS, you also have to re-install your data including all your documents, photos, email, and the like.
You have backed up you computer, haven't you?
Good. I'm glad to hear it.
PS In case you are one of those people who has not backed up his system, take a look at the following 2 articles in this blog, Cloud-Based On-Line Backup for free (up to 2 GB), and Bring in the Clones, how to cheaply backup your system by cloning the hard disk automatically.