The My Documents directory location is stored in the registry at HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders in a key called personal.
The following script is what I use to remap this to u:/documents:
;; create some dirs: u: mkdir documents mkdir outlook ;; Move My Documents folder: ;; delete current setting reg.exe delete "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v Personal /f ;; create new setting reg.exe Add "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v Personal /t REG_SZ /d u:/documents