DOD:S Trial won't work

[Mutley]

Every time I join a server it says either "Client dropped by server" or "Session package something already in use".

Every single server, I tried restarting PC/Steam, no luck. I guess I downloaded a few bad files for it?

 

[Hectic Glenn]

Possible, have you tried verifying integrety of game cache? Right click DOD Source, go to properties and click that button, and it should fix any problematic files. Did you pause and resume the download frequently?