1st Method:
1. Right-click on My Computer icon on Desktop and select "Manage", it'll open "Computer Management" window. ( You can also open it by giving "compmgmt.msc" command in RUN dialog box or Startmenu Search box).
2. Goto "Local Users and Groups -> Users". In right-side pane double-click on "Administrator" account. It'll open its Properties. Deselect "Account is disabled" option and Apply it.
2nd Method:
1. Type "secpol.msc" in RUN dialog box or Startmenu search box, it'll open "Local Security Policy" window.
2. Goto "Local Policies -> Security Options".
3. Double-click on first option "Accounts: Administrator account status" and select "Enabled" and apply it.
3rd Method:
1. Click on “Start button -> All Programs -> Accessories“. Right-click on “Command Prompt” and select “Run As Administrator“. If you are prompted to enter password, enter the password and continue. You can also open Command Prompt in Administrator mode by typing “cmd” in Startmenu Search box and press “Ctrl+Shift+Enter“.
2. Now provide following command:
net user administrator /active:yes