Log out and once at the login prompt, Ctrl + Alt + Delete Twice then type administrator and the password if there is one. If there's no password just hit enter. I'm assuming your xp pro, right click My Computer and select Manage and go to local users and groups double click you're account and click Member Of and see if administrator is there. If it's not add it. If you're XP Home, follow the same loin procedure and go to the control panel and User accounts, open you're account and click Change My Account Type and see if you're administrator. If not select admin. If you're system is locking, it could be a number of things from power supply, memory, heat issues to conflicts.
If you're running XP for four years without an clean install it would be best to do so to have it run as it was new. The four CD's you created will do this by booting from CD #1 and inserting the remainder CD's as requested in the Process of the install.