I bought my copy online 3 months ago, and everything installed fine and was validated as genuine. Now as of Sunday night i'm getting errors saying it isn't. Most resent change was installing Microsoft Security Essentials (Saturday night) with also said my copy was genuine...at the time. Now it agrees with the rest of the system. I tried reentering the product key and got error code 0xC004C003 "the activation server determined the specified product key was blocked".

Have a look to this link, it is about the error code 0xC004C00: http://social.answers.microsoft.com/Forums/en-US/w7install/thread/2c11a69e-c2af-41ab-b691-6fd4e2e13e79 If it does not give you help, I recommand to you to post your problem here: http://social.answers.microsoft.com/Forums/en-US/w7install/threads It is the correct forum for a such problem. Remark: You can proceed by restoring you system state from a restore point and check if your problem is solve or not. Best regards.