The security log on this system is full. Only administrators can log on to fix the problem

This issue occurs because no more events can be logged to the Security log on the computer. When events cannot be written to the Security log, only accounts that are members of the Administrators group can log on to the computer. 

To resolve this issue, use an account that is a member of the Administrators group to log on to the computer. Then, follow these steps to specify that Security log events can be overwritten:

1. Click Start, point to All Programs, point to Administrative Tools, and then click Event Viewer.
2. Right-click Security, and then click Properties.
3. In the Log Size area of the Security Properties window, click the Overwrite events as needed option under When maximum log size is reached.
4. Click OK.
5. Close Event Viewer.