Question : Problem: Error: Com Port In Use

Is there any application that can report what is using/holding a com port. Windows 2000 or XP

Answer : Problem: Error: Com Port In Use

Download Sysinternal's Process Explorer ( http://www.sysinternals.com/ntw2k/freeware/procexp.shtml ) and run it.
Select Find / Find Handle, then Enter "\device\serial" as search string (without the double quotes of course). This will show which process(es) are holding COM ports. \device\serial0 equals to COM1, \device\serial1 is COM2, and so on.