1. More information would be helpful. See
herePlease include whether this is Windows XP Home Edition or Professional, and the name and version number of the third party antivirus (or security software) that you installed.
2. Have you tried booting to Safe Mode (to see if you have access from there)?
Tap F8 key about once per second (or quicker) as the system powers up (or possibly restarts). Windows Advanced Options menu should appear. Use up/down arrows to select regular "Safe Mode" and hit Enter key.