Could this be true? Very doubtful.
Those temeperatures are considerably above that of boiling water The CPU would have cut out before now and the motherboard would have burned out.
As far as Sisoft Sandra is concerned, I wouldn't even believe it if it told me my nickname on here was -pops-. I've often called this particular software panicware or scareware. Quite a good illustration of it doing that.
Have a look in your motherboard manual. It should tell you how to access your BIOS to get the real temperature. If it does turn out to be as high as that, then is the time to get worried.