If you do PIC's (or other MCU's) that's the easiest, probably cheapest, and accurate enough (0.1% enough? that's easy to get) method.
Just use the timer function to count a single period (detect the zero-crossing with a comparator to start/stop the timer).
Then send this via an RS232 level convertor to the PC.
Signal source from a transformer of course (essential for isolation, not just lowering the volts), maybe plus resistive divider to get the volts down for your comparator.
FoxyRick.