You set this in a HIVE*.INF file. This is one i use under Windows 2K, the directory names are set to match Vista (one thing vista got right was to shorten the names to allow for scripts to work. Scripts are not partial to spaces! This changes the locations of the profile directory, all users and default users on setup, and should work under Windows PE.
CODE
; HIVESFT.INF
[AddReg]
; hivesft.inf
HKLM,"SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList","ProfilesDirectory",0x00020002,"%DEFAULT_PROFILES_DIR%"
HKLM,"SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList","AllUsersProfile",0x00000000,"%ALL_USERS%"
HKLM,"SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList","DefaultUserProfile",0x00000000,"%DEFAULT_USER%"
[Strings]
; Shell Folders
DEFAULT_PROFILES_DIR="%SystemDrive%\USERS"
DEFAULT_USER="Default"
ALL_USERS="Public"
Alternately, boot into safe mode or something, and dink these registry entries. You could use tweakui and reboot.