Just for those who might encounter a similar problem, here's what I discovered...

Before I upgraded my network to wireless with a router, I was using a software firewall (Zone Alarm). Of course, I disabled ZA after installing the new network (the router has its own firewall), but it left behind a "hidden" service called True Vector that was running in the background. This didn't seem to affect most network traffic, but on the Win XP Pro machine with WinSCP, it apparently was blocking FTP access.

Once I disabled the True Vector service and fully uninstalled ZoneAlarm, I was back in business.

Whew!

I just installed a wireless network in which three PCs are sharing a DSL connection behind a router. WinSCP is working fine on two of the computers (one is running Win98SE and the other WinXP home), but the third (running WinXP Pro) is unable to make any FTP connections - I get a "Network Connection Refused" instantly whenever I try to connect to any of my remote web host servers.

Now I know this may appear to be a Windows configuration issue, but here's the rub - other FTP clients (such as WS_FTP) are able to connect on this same PC, so it's something unique about WinSCP and this particular machine/OS. I have the exact same settings on this instance of WinSCP as on the other working PCs.

Any ideas of where I should go from here?