Then I have to reboot, and recheck my config:
minerdOC.exe --freq=800 --gc3355=\\.\COM5 --url=stratum+tcp://ltc.give-me-coins.com:3334 --userpass=weminedead.1:x
Have you verified that the Gridseed is still on COM5 after reboot? Unplug it, check Device Manager, re-plug it, check again which COM port appeared.