I found this document very useful. What I use most often is to make a shortcut on my system tray with a target of "%SystemRoot%\explorer.exe /e,c:\". That way, it doesn't open to stupid "My Computer" -lee
Parameters are separated by commas. Many combinations are allowed, but only a few examples are given.
Explorer.exe c:\                Open directory as a single pain of icons
Explorer.exe /e,c:\             Explore drive as 2 lists - 
                                  directories on left & files on right
Explorer.exe /e,/root,c:\       Explore drive without showing other drives
Explorer.exe /n,/e,/select      Opens showing only drives
Explorer.exe /e,/idlist,%I,%L   From Folder\..\Explore in the registry
                                  %I - ID number
                                  %L - Long filename
Explorer.exe  /e,DriveOrDirectory
Explorer.exe  /e,/root,directory,sub-directory
Explorer.exe  /e,/root,directory,/select,sub-directory
| /e | List (explorer) view, Show large icons if missing (Open view) | 
| /root | Sets the top level folder. | 
| /select | Specifies that the directory should be selected without displaying its contents. | 
| /s | ???? | 
| /n | Do not open the selected directory, no effect on NT | 
| /idlist,%I | Expects an ID/handle. May help with cacheing. By itself, opens the desktop as icons. | 
| /inproc | Stops display of window (I don't know why this is useful) | 
Windows Explorer contains 7 icons. In 95 and NT, the 3rd and 4th are different. Use
I have not found a single reference explaining all the options. Some parameters were found by searching the registry, some are from the sources below, and a few are from browsing the executable.