You don't say what PIC it is, you don't to say to which pin the serial data input is connected and you don't say at what frequency the oscillator is running but I'm guessing from your calculations that it's 4MHz...

If you preload the TMR0 with 158 it will interrupt after 98 ticks because it interrupts on overflow from 255 to 0. With a prescaler of 1:8 and running at 4MHz that would be 98*8=784us + the time it actually takes to reload the timer.

As for the serial input I'm going to guess that the PIC has a USART which is taking care of that part.
Finally, there's actually atleast 10 bits to each byte transmitted, 1 startbit, 8 databits and 1 stopbit. So 1/2400*10*4=16.67ms.

/Henrik.