Opened FTP ports for all computers

Recently, I found all the computers on my network has the same problem; FTP port is being created on all type of OS, like Windows 98,ME, 95, 2000, NT4 Server, and Windows XP Pro. Scanned with antivirus program and trojan scan, no problem were found.

3Com software has detected the utilization of the FTP port of all infected computers is very high. What should I do? Please help!!!