This program can convert temperatures between the C,F,&K scales. After the conversion is made, it pauses, clears the screen, and then restarts main() for a new conversion.
Nice job. Now put the menu in a loop and add an option to quit. Once you do that, why don't you try and take a string as input and detect what scale the user entered. For example, 32F would be Fahrenheit, 0C would be Celsius and 273K would be Kelvin, then just convert to the other scales that weren't entered. Also, try and not use system(), its slow, a security risk and most anti-virus products will flag your program as a virus.