The password is Case Sensitive.
Check Caps Lock and try various of Capital combinations.
http://support.microsoft.com/?kbid=321305
Or
Reboot to Safe Mode, select Administrator, leave password blank.
Then go to User Accounts in Control Panel.
Select the user and change the password.
If you are using Windows XP Pro and have encrypted data, you may
permanently lose access to the encrypted data.
Here are other options:
http://www.wininternals.com/products/repairandrecovery/index.asp
http://www.sunbelt-software.com/product.cfm?id=265&affid=wxpnews
Another option is to install Windows XP on another partition and take
Ownership of your data.
You could also perform another installation on the same partition,
although this is not a permanent fix and it is recommended to perform
a Clean Installation after retrieving your data.
After this is resolved, take precautions to not lose the password.
