Microsoft has written Knowledge Base article Q234048 on the subject. I am quoting the beginning of the article so the search engine can find it:
"In Windows 2000, all drive letters assigned to volumes are assigned by the Mount Manager (MountMgr) program. After a drive letter is assigned to a volume by MountMgr, the drive letter is reserved for the volume in the MountMgr database located in the system's registry. Windows 2000 Setup also uses MountMgr to handle BASIC, Dynamic, and legacy FT-Set drive letter assignments during a clean installation and are displayed and assigned in a specific order, sometimes based on past drive letter assignments."