You follow below steps to login as administrator in Windows XP.
Steps:
1.Click on START menu.
2.Then click on Run and TYPE “Regedit.exe” in run text box.
3.After that click on HKEY_LOCAL_MACHINE.
4.Then click on SOFTWARE.
5.Then click on Microsoft.
6.After that click on Windows NT.
7.Then click on CurrentVersion.
8.Then click on Winlogon.
9.After that click on SpecialAccounts.
10.Then click on User List.
11.After that in Registry Editor WINDOW right SIDE double click on Administrator.
12.Then set Value data 1 then click on OK button.
13.Then Exit the Registry Editor.

You follow below steps to login as administrator in Windows XP.
Steps:
1.Click on Start menu.
2.Then click on Run and type “Regedit.exe” in run text box.
3.After that click on HKEY_LOCAL_MACHINE.
4.Then click on SOFTWARE.
5.Then click on Microsoft.
6.After that click on Windows NT.
7.Then click on CurrentVersion.
8.Then click on Winlogon.
9.After that click on SpecialAccounts.
10.Then click on User List.
11.After that in Registry Editor window right side double click on Administrator.
12.Then set Value data 1 then click on OK button.
13.Then Exit the Registry Editor.