You can't, you need to wait for it to load, then logoff. Then, at the
welcome screen, if the admin account is hidden hit ctrl+alt+delete twice to
bring up the older Win2000 style screen where you can type it in.
What do you want to disable about ctrl+alt+delete? On a Pro system,
start/run gpedit.msc and expand User Config/Admin templates/system/cad
options.