The settings under Preferences / Folders / Auto-Loading let you tell Opus to not automatically read folders from certain drive types when opening new windows (until you click on something to tell it to go ahead and read the folder).
Alternatively, if you do not want Opus to remember the folders which were open the last time it was used, and would prefer it to always open specified folders that never change and are always there (e.g. C:\ or Desktop), the short Startup Folders guide explains how to do that.