I'm not familiar with the new Ivy Bridge type BIOS but I'm wondering if this isn't some sort of configuration step you've missed. I'm presuming you're still able to run the PC fine by ignoring the error or acknowledging it but you didn't say what it is you do when you get this. In any event, I'd look to see what ASUS utilities you might have installed with the CD that came with it. Since it says "specified OS" this sounds like the something that a builder would configure either in the Ivy Bridge setup or some ASUS utility that runs at startup. You could try the usual by disabling startup items by running msconfig or revisiting the BIOS setup for some missed step. Good luck.
I just finished building my new gaming computer and it is working beautifully for the most part except for some software issues, every time I boot up as soon as I log on i get an error that pops up saying: "can't find specified OS version and language" with the title AsusSetup on the popup box. I have had this issue since the start.
2x XFX Radeon HD 6970 (Crossfire)
Intel Core i7 3.4GHz Ivy Bridge
16GB Ram 1600 MHz
ASUS P8Z77-V LGA 1155 MoBo
Majesty ABS 1100 watt PSU
Antec DF-85 Case
500GB 7200RPM Hard Drive (+1TB soon)