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

In Windows XP, when a user who is not a member of the Administrator Group try to log on to a computer. They might receive the following message in the “Log On to Windows” window:

This problem 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. You have a few options for solving this issue, you can clear the log entirely, you can expand the amount of space the log can take up, or you can set the log to overwrite old events.

Option 1 – To clear the log even

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 left pane, right click on SECURITY and then select CLEAR ALL EVENTS.

Option 2 – Increase the size of the Security log or configure the log to overwrite as needed.

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 Security Log file properties, the default setting are show below:-
Maximum log Size – 512KB
Overwrite events older than 7 days

4.To increase the amount of space for log size, just change the maximum log size value to the higher log size value.

Eg: set to 5000KB

5. To configure the log to overwrite as needed,  just click the Overwrite events as needed option under When maximum log size is reached

4. Click OK.

5. Close Event Viewer.
To prevent this problem from happening, the best option is to overwrite the security events as you needed. But the oldest events will be lost.