Boot from your XP CD and press "any key" when asked. This will not overwrite your files but any windows updates will have to be redone.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)
If after the reinstall you still can't enter windows then redo the above but use the XP CD to delete partition, format and clean install.
HOW TO: Partition and Format a Hard Disk in Windows XP (Q313348)
Read the paragraph,
"How to Partition and Format Your Hard Disk by Using Windows XP Setup."
Afterwards go to Start > Run and in the Open box type "control userpasswords2", less the quotes.Select the Advanced tab in the User Accounts window.Press the Advanced button below the Advanced user management header.Select Users in the Local Users and Groups. In the right pane, right-click the user name for which you want to change the setting, and select Properties. On the General tab, check Password never expires. Click Apply and OK.
Before you forget your passwords do this.
To create a password reset disk. (See Help and Support)
The Forgotten Password Wizard helps you create a password reset disk that you can use to recover your user account and personalized computer settings if you forget your password.
If you have a computer administrator account
Open User Accounts in Control Panel.
Click your user name.
Under Related Tasks located on the left side of the window, click Prevent a forgotten password.
In the Forgotten Password Wizard, follow the instructions as they appear on the screen.
If you have a limited account
Open User Accounts in Control Panel.
Under Related Tasks located on the left side of the window, click Prevent a forgotten password.
In the Forgotten Password Wizard, follow the instructions as they appear on the screen.