Just making a quick post to help out any Googler's out there looking for the solution to this problem. I had a Dell SC1425 pizza box server with an error/alarm light on it's face recently. The alarm showed up when the server had been rebooted after a full lockup. I downloaded the Dell DSET utility and ran a report, which found this error:
PROC Machine Chk processor sensor transitioned to non-recoverable
Basically this means that the mainboard detected that the processor stopped responding (i.e., locked up), which of course we already knew.
To get rid of the alarm light, just run DSET again and tell it to clear the log. After you do so, your server lights should once again be a calming, cool blue.