The first (and easiest) thing to check out is this:
XP creates a default account for a user named "Administrator", and this account is different fron any other user accounts which are members of the "Administrators" group.
You'll sometimes find that although passwords have been set for all other user accounts that have been created on the computer, there has been no password set for the user "Administrator". Try typing Administrator as the user name in the login window and then just hit Enter; if no password has been set for the Administrator user account, Windows will happily log you in as the system administrator. If that works, you can modify your user accounts/passwords from there.
If that doesn't work, things get difficult; you can't just "bypass" the login on XP or 2000 systems like you could with Win 95/98.
There are a number of tools you can use to recover or change a forgotten password, and some of them involve a Linux boot disk. The reason being that Linux can access Windows drives/partitions, but it totally ignores Windows permissions and passwords.
Here are a couple of links which discuss some of the options:
http://www.petri.co.il/forgot_admin...or_password.htm
http://is-it-true.org/nt/atips/atips262.shtml
And here are the results of a general Google search on the subject:
http://www.google.com/search?hl=en&...ecovery&spell=1