What you ought to do is find out what chip is inside the serial<>usb adapters that work, saying its a belkin might not work since belkin changes between prolific and ftdi etc, either use the drivers loaded for the working ones and determine the maker from the usb pid/vids or the driver names, or look on the chip itself.
I usually have more luck with the FTDI than the prolific with most of the legacy devices.
USB<>serial adapters do not truely recreate RS232 which is the base of the problem, some of them simulate RTS/CTS and others include the hardware to do it, find out if the power to win software is using flow control, or just dumping the data with no flow control, thats usually what they do but its less accurate and leads to more problems, changing the baud rate can help as the usb<>serial adapters all have timing problems.
there are lots of places you can buy rs232 enabled laptops for cheap thats what a lot of people do when they're stuck with RS232. This place in vegas has a bunch
LaptopXchange™ - We Buy Sell Trade Service & Upgrade Laptops & Computer Laptop Franchise Information theres a place in socal that sells them too, i forget the name, wierd stuff in san jose area might have some too, but they're fairly easy to find over the web. the older IBM thinkpads are really useful here.
just make sure you get one that comes with or can be upgraded to a version of windows the software needs.
also there are pcmcia cards that have serial support, they usually better but are very expensive.
so find out if it needs flow control and match the adapter, try a different baud rate if possible and check who makes the inteface chip, not who packages and sells it.