Try starting in Safe Mode. Is there an additional account named "Administrator" or "Owner" on the screen when in Safe Mode? If so, try that with a blank password, for starters.
Otherwise, if you have a proper WinXP CD (as opposed to a Restore CD from Compaq/Dell/Gateway/etc), then you could do a Repair-installation of Windows:
1) start Windows Setup from CD-ROM
2) when you're first allowed to choose Repair versus Install, choose
Install.
3) Windows Setup proceeds for a couple minutes, you agree to the EULA, and it shows the existing disk partitions. Choose the C:\ partition and Windows Setup should see the existing C:\WINDOWS directory and offer to repair that installation.
NOW choose "Repair."
4) Make sure your network cable is unplugged, or your wireless access point and/or modem are turned off, to prevent direct worm attacks of your "raw" Windows installation. Before connecting the computer to a network, enable the Internet Connection Firewall (consult Windows Help if you don't know where) or install
Service Pack 2 from a CD (which enables the firewall, called "Windows Firewall" on WinXP SP2, and patches many vulnerabilities too).
5) when your repair-installation of Windows is done, you will have your data and programs still in place, but you'll need to reinstall your service packs and patches for Windows. Visit Windows Update and
Office Update several times until you come up clean.