Are you using XP Home or XP Pro?
I dont know it the Pro/Home difference will matter much, or alot, but here goes.
Note please that this COULD result in you losing and encrypted documents, if you have made folders private.
Click start > Run.
Type compmgmt.msc in the box and push enter.
Under the local users and groups folder in the tree directory on the left, select the users folder.
You can then right click the user you want to set a password for, and select Set Password.
Hope this helps
Kurtis