What is windoes doing?

[dwight via cctalk]

I have a question. I use the USB port for serial. In my program, I use a fixed 
com port. When going to the control panel, I find that I see (in use) tags on 
some of the com ports. I'm the only one currently using the com ports but 
recently another (in use) showed up, requiring me to modify my program to use 
another com port. How does one unuse a com port? how do I find out what is 
using it so I can stop it? I'm using windows 7 professional. Has anyone else 
had this problem?
Dwight