Bart
The simple 2 transistor interface works with Windows 95 to ME but is not compatable with W2000 or XP. This is because it uses the 'ring indicator' pin rather than normal serial transfer. MS have dropped support for this pin in their later systems.
The PIC Serial interface translates the transmitter pulse train into real bytes of serial data transmitted by normal methods. Hence the PIC version is compatable with all systems. I also recon that the PIC version is slightly smoother. The 19,200 baud PIC version is able to send the data for one pulse during the first 1ms of the following one. It can then concentrate 100% on getting an accurate reading the next - and so on.
The IQR serial (and Parallel) interfaces rely on the PC clock for time measurement. If the PC is distracted (higher priority interrupts) it will make a slight error. Nevertheless if you have W95 - ME and a soldering iron I recommend you make up the IRQ version. I have circuit diagrams and transmitter connection details on my website. And if all else fails I'll be happy to supply you a PIC version!
Happy flying
Mike