Using FTP to access windows files


Hello everyone
I am new in the mac world, i just go a powerbook few weeks ago and I decide to stop using windows, but I also decide to leave all my file on the windows machine and access them via ftp. I can connect to my windows machine see all my files and open any file, but if I want to save any document to my machine via ftp using my laptop it's say that my folder ftp client can't be modified . I try doing the same thing using another pc and it did work. I just wondering what should I do. I need you help, thank you in advance...