@mbromham
There can be a few causes for that type of behaviour.
As a test only clear all your temporary files and caches (I use CCLEANER)
Restart your computer and ensure your AV is OFF completely !
Plug the board into a USB 2.0 port and check which port if gives out.
Start the IDE and upload a simple sketch.
Unplug the board from the computer and check the com port went away.
Plug the board back in and see if it changed again.
If it did then you ruled out your AV as a cause.
You should then start to check and see if any other software is causing the issue as some alternate IDE's and similar can also cause such issues.
You can use the same steps as above but ensure you turn off/disable the software you think may be a problem via task manager or similar.
If it continues you may also need to reset the COM stack.
if you are using windows then
this link provides a few methods of doing that..