The Windows Explorer default folder is the folder that is shown when Windows Explorer is started. This is the My Documents folder if you use Windows XP or the Documents folder if you happen to use Windows Vista. While most users might find it comfortable that those are the default folders others prefer to work with different folders and largely ignore the My Documents or Documents folder.
I happen to be one of them and found a way to change the default Windows Explorer folder. To do that you have to open the Windows Explorer Properties and the easiest way to access those is by opening the Start Menu, All Programs, Accessories and right-clicking on the Windows Explorer entry there. Select Properties from the menu that pops up which should display the windows Explorer Properties.
Select the Shortcut tab if it is not selected yet and locate the Target field in this menu. It should display %SystemRoot%\explorer.exe. Append /n, /e, path to it so that you get a string that looks like this: %SystemRoot%\explorer.exe /n, /e, path with path being the hard drive and folder that you want to display as the default one in Windows Explorer. (e:\ for instance)

You can test the changes right away. If everything was set the right way Windows Explorer should now display the folder that you have added in the Target field instead of the default folder.
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Don’t fight with Explorer, replace it. Somewhere in the range of free A43 and xplorer² and pricey but highly geared Directory Opus is a better product, easier made to behave as one desires.
Works, but does anyone have a way to select the default directory when you right click on the start menu and select Explore from there… Because this does not affect that… If you have an answer, please email me… Been trying to get this right for years…
Guess most have moved on, but it buged me too
from someones post is the detail I pasted as a guide
The last line works for C;\
When you right-click the Start button to open Windows Explorer, the default selection is a deeply buried user directory under C:\Documents and Settings. My organization is networked to a central file server that stores all our files. We wanted users to default to the server rather than their local computers when they used Windows Explorer.
So, we edited the HKEY_CLASSES_ROOT\Folder\shell\explore\ddeexec registry subkey. If you change this subkey’s default value to [ExploreFolder("yourNetworkDrive", yourNetworkDrive, %S)], you force Windows Explorer to start at the drive you specify. Replace yourNetworkDrive with the drive you want. In my case, the drive is mapped as S: on all workstations, so the string on my system is: [ExploreFolder("S:\", S:\, %S)]. To present a single-pane Windows Explorer view, you can change ExploreFolder to ViewFolder.
I created a .reg file to automatically change the default Windows Explorer directory. Listing 1 shows this file. You need to change the two S: entries in the file to match your network’s mapping (leave the %S entry as is).
[ExploreFolder("C:\", C:\,%S)]