cannot save files to Windows server 2003 file server from MAC

Hi, 

We have few MAC users and others mostly windows based users. Problem now is MAC users cannot save any files to the file server. I had the issues to save in the portable hard drive then i format the HDD to fat 32 and now i can save, But the file server cannot be done like that. 

What i can do to save the files to file server? please advice me

thanks 

saththiyan