I use a USB to serial adapter in my setup, which includes a PC that doesn't have a serial port, for more than a year. Zero problems. Got it for $30 I think. And I used to have the USB version of Weatherlink that did have tons of problems before I switched to a serial connection. I suppose you could see if you could buy one, try it, and if it doesn't do the trick get a refund and get the serial card.
Cheers,
Andreas