If the PC in question has USB but not serial then what about a USB to serial convertor cable? .
If you are really stuck for such a PC then PM me. I might be able to help (with a low spec PC that is capable of running Win98 and has a serial port but you'd have to supply Win98 yourself) but it might take me a while to get it together. If it is going to a good cause then this would be free gratis.
I could have one for you at no charge after the first week in October. I'm busy building a new PC for my father-in-law and we'll be visiting Ballydehop, West Cork to help him with the setup. He's running a dinosaur with WIN98, so I'm pretty sure it'll have a serial port. I live in Maynooth and my e-mail is connolly.paddy@gmail.com.
I have a Dell Optiplex GX1 with one ISA and (I think) 2 PCI slots. It's spare but I can't recall the spec offhand. Probably a Pentium II (or III?) 350Mhz with c. 128MB RAM and 20GB HD. It did run Windows Me at one stage but was upgraded to Xubuntu and is probably "blank" right now. PM me if you are interested.