It could be you need to alter tha cable wiring for XP... NT/2K/XP pro may need the some of the other serial cable signals handled differently to work.
I don't have allot of detail on XP handy. I used to have to do stuff like that when making serial cables for plotters printers and other junk years ago. You couldn't just use the data lines and ground. Sometimes it was as simple as putting a jumper across a couple of the com port terminals so it would see one or more handshake lines go active.
My guess is that XP is doing something like that. Until it sees the handshake lines it's looking for it just ignores the data line.
Serial port wiring is pretty standard. There are specs for it all over the place. I don't have a Winaldl compatible car so I can't test the thing to see what is going on.
It is also posible the author needs to alter the program to handle the NT/2K/XP com ports.
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