Well, this is a VERY common problem. Some get this warning 2 days after they got the laptop. Many say they sent it back, but no real definitive answer to whether it is a hardware issue, or a detection issue. Google the phrase: Toshiba Warning: a problem with the cooling system has been detected. Please turn off the computer immediately, and return it for service.
You'll see what I mean. Also see this Toshiba Forum thread: Cooling Warning - Toshiba Forums
You say your laptop is under warranty? I would contact Toshiba support, or post on their forum for your country. With such a common error, and many reporting temps are ok, and posts going back almost 2 years, you'd think they would know the cause by now. Sorry, I looked through a number of sites, and found no definitive fix. A Guy