In the past Microsoft have been fairly lax about people using pirated copies of Windows or Office.
There was little they could do about people sharing validation codes on the internet so you could install a pirated copy of Windows or Office.
Then a few months ago they created a program called Genuine Windows Advantage which was downloaded as part of their regular downloads.
It now checks your validation code, and if it is one that has been used by many people they assume it is a pirated copy of Windows and give you this warning.
Eventually if you do nothing Windows will stop working.
I am not saying you DO have a pirated copy, but Microsoft seems to think you have.
Read more here:
http://www.microsoft.com/genuine/default.mspx? displaylang=en
http://www.microsoft.com/genuine/downloads/Why Validate.aspx?displaylang=en