You can cause event viewer to overwrite events as needed so that they won't fill up and prevent you from logging in.
To apply this policy on a global domain scale:
-open the Active Directory Users and Computers snap-in
-open your domain's Properties dialog box, and go to the Group Policy tab
-Select the Default Domain Policy GPO, and click Edit
-In the resulting window's left pane, go to Computer Configuration, Windows Settings, Security Settings, Event Log, Settings for Event Log
To apply this policy on a global domain scale:
-open the Active Directory Users and Computers snap-in
-open your domain's Properties dialog box, and go to the Group Policy tab
-Select the Default Domain Policy GPO, and click Edit
-In the resulting window's left pane, go to Computer Configuration, Windows Settings, Security Settings, Event Log, Settings for Event Log