if you want to even not enter a pass:

1. Click Start, click Run, and type control userpasswords2.

2. Clear the Users must enter a username and password to use this computer check box.

3. Click Apply.

4. Enter the user name and password you wish to automatically log on with, and then click OK.

5. Click OK again and you're all done.

Auto logon using regedit:
# Click Start, click Run, type regedit, and then click OK.
# Locate the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
# Using your account name and password, double-click the DefaultUserName entry, type your user name, and then click OK.
# 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.

If no DefaultPassword string is specified, Windows XP automatically changes the value of the AutoAdminLogon registry key from 1 (true) to 0 (false) to turn off the AutoAdminLogon feature.
# 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.

# Quit Registry Editor.
# Click Start, click Restart, and then click OK.