1) Hook the pulse train up to a serial port line or parallel
port line and do the timing on the PC. There are Linux and
Windows drivers for both of those approaches. Schematics
for the H/W interface can be found in various places on the
web, like this:
2) Buy or build a "cable" that contains a microprocessor (it
usally seems to be a PIC of some flavor) that doest the
timing and sends a data stream via USB or RS-232. There are
a bunch of these to choose from as well: