Hi billyfromhill, and welcome to the VMware Communities!
Which model of USB-to-serial converter are you using? It may be worth checking the vendor's website (or checking in Windows Update on your Windows 7 host) to see if there are updated drivers available. The particular message you are seeing should only occur when the host's serial port driver behaves unexpectedly and confuses VMware Workstation to the point where it can't continue to use that host port.
Cheers,
--
Darius