How to turn on automatic logon in Windows XP

http://support.microsoft.com/kb/315231

You can use Registry Editor to add your log on information. To do this, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate the following registry key:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
3. Using your account name and password, double-click the DefaultUserName entry, type your user name, and then click OK.
4. Double-click the DefaultPassword entry, type your password under the value data box, and then click OK.
   
   If there is no DefaultPassword value, create the value. To do this, follow these steps:
   1. In Registry Editor, click Edit, click New, and then click String Value.
   2. Type DefaultPassword as the value name, and then press ENTER.
   3. Double-click the newly created key, and then type your password in the Value Data box.
   Note: If the DefaultPassword registry entry does not exist, Windows XP automatically changes the value of the AutoAdminLogonregistry key from 1 (true) to 0 (false) to turn off the AutoAdminLogon feature after the computer is restarted.
5. Double-click the AutoAdminLogon entry, type 1 in the Value Data box, and then click OK.
   
   If there is no AutoAdminLogon entry, create the entry. To do this, follow these steps:
   1. In Registry Editor, click Edit, click New, and then click String Value.
   2. Type AutoAdminLogon as the value name, and then press ENTER.
   3. Double-click the newly created key, and then type 1 in the Value Data box.
6. Exit Registry Editor.
7. Click Start, click Restart, and then click OK.

After your computer restarts and Windows XP starts, you can log on automatically.