|Enable in bios to be sure.|
Set speed and connection type in device manager, or set mode for dos if dos access.
They make simple loop back test adapters or you can make one yourself.
Otherwise you can test null modem computer to computer. You could even test back to second com port or even lpt port.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10