Have you only just checked the CPU temp under load? If so, its probably always been 100*C. 7700K's run hot anyway, combine one with a stock Intel cooler and this is what you get.
Not that it matters but... Why did you buy a 7700K if you're just going to run the stock cooler? The whole purpose of the 'K' chips is that they are overclockable.
Just getting a better cooler is probably a better idea than doing what is suggested above, unless you feel confident in doing it. Both will result in much lower temperatures...
