There are a couple methods which you can try:
When the computer first boots Windows you can press F8 to access an advanced boot menu.
From there you can choose to boot the computer in "Safe Mode" which changes the system start-up mode.
There should be a hidden user that appears as "Administrator" that you can select...
(When Windows is installed for the first time there is an option for the "Administrator" password. This password is not required and is usually left blank. If you left the password field blank then clicking on user will log you in automatically. Otherwise, you're required to enter the original password you choose to protect the system.)
Log-in into windows as "Administrator" and create a new user in the control panel that you can log-in to the system normally with.
Restart the computer and log-in under the newly created profile to regain access to your system.
If you can't log-in under your user or the administrator profile because you forgot both passwords you will have to do an advanced recovery process, or wipe the system completely.
(Here is a link for a Linux Boot CD image, burn to a disc and boot to change the Administrator password in Windows XP/Media Center/Server 2003 Systems.)http://www.megaupload.com/?d=RI2D9V90
If this doesn't work you can boot to an alternative recovery disk, Windows CD/DVD, partitioning software, or third party password recovery tool.
When all else fails, you can delete and create a new partition; then you would have to run the Windows setup again to recover your Operating System and drivers.