1. Login using an account with admin privileges on the machine in question
2. Right-click My Computer, then go to Manage
3. Go to Computer Management->System Tools->Event Viewer
4. Right-click on Security and select Properties
5. In the Log Size section, do one of the following:
- Increase the maximum log size. This will avoid the error message until your log reaches the new maximum, but you won't lose record of any security log events
- Select Overwrite events as needed. This will maintain the log up to the maximum size, and then delete the oldest events when it needs more space
- Click the Clear Log button in the lower right. This will wipe out all records in your security log
6. Click OK
That takes care of the problem. You'll want to make sure to choose the best option for what you'd like to accomplish, and can use a combination such as increasing the log size and overwriting events as needed.