1.
Click <Start> <Settings> <Control Panel> <Network>

2.
On the <Configuration tab>, click 'Windows Logon' in the 'Primary Network Logon' box, and then click <OK>.

3.
When you are prompted to 'restart your computer', click <No>.

4.
In <Control Panel>, double-click <Passwords>.

5.
On the <Change Passwords tab, click 'Change Windows Password', select any of the check boxes that you want, and then click <OK>.

NOTE: If you cancel the network logon dialog box when you start your computer, the Change Passwords tab may not be available. You must log on so that the Change Passwords tab is available.

6.
In the Change Windows Password dialog box, type your current Windows password in the Old Password box. Leave the New Password and Confirm New Password boxes blank, click OK, and then click OK.

NOTE: If you have forgotten your old password, view the following article in the Microsoft Knowledge Base:
189126 Microsoft's Policy Regarding Missing or Invalid Passwords

7.
On the User Profiles tab, verify that the All users of this PC use the same preferences and desktop settings option is selected, and then click Close.

8.
Click Start, point to Search (or Find), and then click For Files or Folders.

9.
Type *.pwl in the Named box, click Local Hard Drives in the Look in box, and then press ENTER.

10.
Right-click one of the .pwl files, click Rename, and then rename the file with an .old file name extension. Repeat this step for each .pwl file.

NOTE: If you do not rename the .pwl files, the passwords from those files may be detected by Windows and the Windows Logon request may continue to appear.

11.
Shut down and then restart your computer.

Visit this site for more info:

http://support.microsoft.com/kb/q152104/