In tip 7873, I explained why the Administrator account might be missing from the Welcome screen.

While performing some tests on a non-domain computer, I discovered that if an account is NOT a member of the Administrators, Power Users, or Users groups, it will NOT be displayed on Welcome screen.

To add an account to a local group, open a CMD.EXE windows and type net localgroup "GroupName" "UserName" /ADD and press Enter.