Event ID 333 Source: Application Popup

Event Type: Error
Event Source: Application Popup
Event Category: None
Event ID: 333
Date: date
Time: time
User: N/A
Computer: computer name
Description: An I/O operation initiated by the Registry failed unrecoverably. The Registry could not read in, or write out, or flush, one of the files that contain the system's image of the Registry.
This event begins recording every 5 minutes after a server restart. Server becomes unresponsive. If server is running Citrix or Terminal Services, users cannot access applications.

Issue indicates that Non Paged or Paged Pool Memory is temporarily insufficient.

Microsoft has issued a hotfix to resolve this issue. You will have to requests the hotfix from Microsoft Support (at no charge) as it is not publicly released at this time. To apply this hotfix, the computer must have Windows Server 2003 Service Pack 1 or Service Pack 2 installed.

Obtain the hotfix and then enable it with the following registry entry:
Location: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager
Name: RegistryFlushErrorSubside
Value: 1 or 2 (If you set the value of the RegistryFlushErrorSubside registry entry to 1, the system only records one event 333 when the temporary memory insufficiency occurs. If the value is set to 2, the system triggers a Stop error for diagnostic purposes.)

Source: http://support.microsoft.com/kb/970054