Real time serial communication

I use serial port to communicate with external hardware.
My application has to receive a byte every 60 millisecond and send repley
before the next byte arrives.
But the application isn't notified quickly enough abut arriving byte. The delay could even be half a second although the thread has the highest possible priority. This happens especially while any other disk operations are performed. I use standard functions to send and receive data and I run it under
windows NT.
Does anybody know how to enforce operationg system (I think here the problem is here)to
notify application immediatelly about serial port events.

Thanks in advance
(ReadFile and WriteFile)