I think I have 3 of those USB-serial converters ( I think I may have posted model info before). The one that works the best for me is the oldest one. I originally started using it on an old Win98 computer I think. It works on that, and it also works with WinXP, but it doesn't work with Win2000 for some reason. I have one that works OK but has the annoying feature that it insists on only using high port numbers, like 6 7, 8, etc, and each time you use it in a different USB port, it assigns a new higher port number, and yes, I went into the control panel and switched it to lower port numbers, and it was still refusing to work down there. Some programs don't let you use high port numbers, so I've had problems using that converter at times. Only use it on programs that accept high port numbers.
But with anything up through Win-XP, these converters should work fine. I suspect that you'd have to fight with Vista though. I helped a couple friends try to set up their Vista systems, and until I can't find hardware that is backward compatable, I'm not going near Vista. I actually had to go back to DOS, to get VISTA to allow us to load some programs onto his hard disk via network. Took us all afternoon to do what would have taken a half hour with any other version of Windows.
OH, and BTW, I use the OLD converter with my Fortec Lifetime, Ultra, Mercury, Diamond9000, and Coolsat 8100, along with a dozen other serial devices. I use serial so much that I wouldn't own a computer if it didn't do serial via some route. I've even re-wired serial devices so that I could send from one device and receive from another device all through the same port. May serial live forever.