SQL Server agent Error I am in a new shop and the below error started to show up on 2/22 repeatedly until 3/7. 

Event Type: Error
Event Source:   SQLSERVERAGENT
Event Category: Alert Engine 
Event ID:   318
Description:
Unable to read local eventlog (reason: The event log file is corrupted).

Event Type: Information
Event Source:   SQLSERVERAGENT
Event Category: Alert Engine 
Event ID:   311
Date:       2/22/2011
Time:       6:52:44 AM
User:       N/A
Computer:   XXXXXXX
Description:
Attempting to re-open the local eventlog...

Event Type: Warning
Event Source:   SQLSERVERAGENT
Event Category: Alert Engine 
Event ID:   312
Description:
Successfully re-opened the local eventlog - NOTE: Some events may have been missed.


Then, the server shut down unexpectedly. The shutdown stopped the events. But, my concerns are: What was happening? Does anyone know what this means? More importantly, what kind of measures do we need to take to prevent this from happening?
Sounds like you've got some sort of corruption problem on the disk which the SQL Agent logs get written to.  I'd check the disk for corruption, as well as the C drive in case it's talking about the application log which the SQL Agent also writes to.