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Reboot your computer and at the Windows XP progress bar press F8 and in the menu that appears select Safe Mode with Networking. Once it boots into Safe Mode try again to login and if successful then click Start, My Computer, then right click drive C: and select Properties, then click the Tools tab then click the Check Now button, and in the box that appears, click to check mark the top box and then click Start. When it asks if you want to run the scan on reboot select Yes then Ok then reboot.
After reboot try logging in again. If you still cannot login then reboot again into Safe Mode with Networking and perform a System Restore to roll back your system configuration to a date before the issue occurred:
1. Click the Start button, then click All Programs, click Accessories, click System Tools, and then click System Restore.
2. If you are prompted for an administrator password XXXXX confirmation, enter the password XXXXX provide confirmation.
3. Follow the prompts to restore your system settings to a time before the issue occurred.