connection refused

my computer works with win98se, ADSL connection. I have a web site which i update daily or several times a week. i use alternatively two ftp programs.
two days ago when connecting through ftp to my host i received an error message #550 saying connection refused.
I checked with my host and they find no fault in their server. i connected also successfully to the server from my home computer by dial-up connection.
today the program refused to connect in the morning, some time later i connected and uploded files, and noew it refuses to connect again.
the problem is probably in my computer but i don't know how to find and correct it.
please advise.
thanks